No good turning back. Well, it will take more than two years, more than five years, because look how long that they talk about it. Weve just got to go in and show them that we do mean business. And well be looking at the Prime Ministers hint that cooperation on security is closely linked to the outcome of the brexit deal. Also tonight, a reminder of the Security Threat here in the uk. A crowd converges on westminster bridge, to remember the victims of the Terror Attack which took place here a week ago. Well have more reaction and opinion to todays historic events, as the Prime Minister triggers britains exit from europe. Well hear from politicians, business and members of the public. Thats tonight, here on bbc news. Good evening. The United Kingdom has formally served notice that its leaving the European Union. A letter signed by theresa may was handed to the president of the European Council today. The Prime Minister told mps she wanted a smooth and orderly brexit, but she also hinted of implications for Security Cooperation if a deal was not reached. And in a potential setback for mrs may. The german chancellor Angela Merkel has warned that the terms of britains divorce have to be settled, before any talks on trade can begin. Our Political EditorLaura Kuenssberg reports on the day that article 50 was triggered. Some moments make us. This is one. The minute in westminster, belfast, edinburgh and cardiff that the United Kingdom formally changed course. The article 50 process is now underway and in accordance with the wishes of the british people, the United Kingdom is leaving the European Union. This is an historic moment from which there can be no turning back. Probably our last ambassador inside the European Union handing over the letter at 12 25pm. The document that says we are on our way out. Theresa mays signature on our departure. Herjob now, to make it work. This, her hope. A country that goes out into the world to build relationships with old friends and new allies alike. And that is why i have set out a clear and ambitious plan for the negotiations ahead. It is a plan for a new, deep and special partnership between britain and the European Union. Her decisions about how, mean we are out of the Single Market to control immigration. As european leaders have said many times, we cannot cherry pick and remain members of the Single Market without accepting the four freedoms that are indivisible. We respect that position. A friendlier tone to the continent, an ambition to bring this country together. And no cliff edge, no abrupt change for business. But mr speaker, when i sit around the Negotiating Table in the months ahead i will represent every person in the United Kingdom, young and old, rich and poor, city, town, country and all the villages and hamlets in between. And, yes, those eu nationals that have made this country their home. And it is my fierce determination to get the right deal for every Single Person in this country. In perhaps the most important letter that shell ever pen, the Prime Minister wrote of her hope to give reassurance quickly to the millions of eu citizens who live here and brits abroad. We should always put our citizens first, we should aim to strike an early agreement about their rights. But no guarantees. The Prime Minister wants a Free Trade Deal with the eu of greater scope and ambition than any before. A bold hope, seen as naive by some, to try to protect firms who do business around the continent from new rules and barriers. No overt threat to walk away but a serious warning a failure to Reach Agreement would mean our co operation in the fight against crime and terrorism would be weakened. We must work hard to avoid that outcome. Her message, the eu needs us. She wants also to agree the terms of our future partnership alongside those of our withdrawal from the eu, to work out how we leave at the same time as sorting out the future. Labour arent the only ones sceptical she can deliver. If the Prime Minister can deliver a deal that meets our tests, that will be fine, we will back her. More than ever, britain needs a government that will deliver for the whole country, not just the few. And that is the ultimate test of the brexit deal that the Prime Minister must now secure. The clock is ticking now. Memories of today will be so different. A public party for some. Even though thats not actually the foreign secretary. And celebrations after hours tonight. Everyone unite as brothers. Yet it was almost a wake for others. Thats the sense in the home of the eu. Theres no reason to pretend that this is a happy day, neither in brussels nor in london. Some powers coming back from brussels will bypass this place and flow to holyrood, cardiff and stormont. But for remainers here and in the Scottish Government those promises dont go far enough. The Prime Minister still cant answer basic questions about what brexit will mean for businesses, the economy generally and for the type of society we live in. This six simple pages will do much to determine our place in the world in the future. The letter is less abrasive in tone to the rest of the eu than when theresa may started as Prime Minister, when she still had to persuade her party she really was committed to leaving. Now the clamour of the referendum is gone, the tone is politely practical. The message of the letter, get real. You need us and we need you. Remember, we bring a lot to the table when it comes to policing, security and intelligence services. And i think thats a little. It wasnt firing a shot but she was just making a reminder, remember what is at stake here. It is an enormous decision. I think it is exciting but i dont underestimate the scale of the task that lies ahead in the next two years. What has happened today is the biggest stimulation of british power and sovereignty in my lifetime. A letter which is really about kicking off a Trade Negotiation had six mentions of trade and ii mentions of security. It struck me as a reckless series of threats. Not that he ever needs a reason to be pictured with a pint, today ukip were celebrating. Over the moon, happy. Today, for me, after 25 years of campaigning, The Impossible Dream came true, im very pleased. And look who popped up later. What matters now is that we make sure we have a successful negotiation and we try to maintain a close relationship between britain and the European Union. In a rare interview inside number ten for the bbc, the Prime Minister promised, despite all the difficulties, our relationship with the rest of the continent will be just as good. What we are both looking for is that comprehensive Free Trade Agreement which gives that ability to trade freely into the european Single Market and for them to trade with us. It will be a different relationship but i think it can have the same benefits in terms of that free access to trade. An assertion that will take a lot to prove. One her counterparts in europe struggle to believe. Number tens time for preparation is up, now time to try to persuade. Laura kuenssberg, bbc news, westminster. Chancellor merkel of germany warned today that sorting out britains Future Trade Deal with the eu would only be possible once the terms of britains departure had been settled. The president of the European Council donald tusk said the remaining Member States would pull together during the talks ahead. And he confirmed he would set out his planned negotiating guidelines on friday, ahead of a Special Summit Next month. With her assessment of the view at the heart of the European Union, heres our europe editor katya adler. The man with the burning letter in his briefcase. Good morning. Big day, ambassador . Sir tim barrow arrived without much fanfare at the European Council building this morning. But this isntjust an historic day for the uk. For the eu, it is a momentous, never to be forgotten kick in the teeth. Visibly unhappy, this was the recipient of britains letter starting the Brexit Process. So here it is. Six pages. Donald tusk, the man who represent all eu Member States here in brussels. There is nothing to win in this process, and im talking about both sides. This is about damage control. European Commission PresidentJean Claude Juncker was also down in the mouth. Im sad. Im deeply sad. But beneath that sadness, palpable resentment among some eu leaders today that theresa may appeared in her letter to link the likelihood of a good trade deal, so hoped for by britain, with continued cooperation on security, so needed by the eu. I think that irrespective of what an agreement can be, what sort of agreement you can do on trade at the end of the day, we remain part of the same family and we should remain committed to fighting terrorism. So what now . Well, the European Commission is the lead negotiator for the eu when it comes to brexit. Frans timmermans is the commissions vice president. But how can negotiations even start, i wondered, with both sides at loggerheads . The uk wants divorce talks and talks of a new trade deal in parallel. The Commission Says non divorce comes first. That is all part of how we negotiate. But how do you square that circle . Everybody starts with his own interest and tries to formulate his own interest in the best possible way. Thats what we all do. So whats the problem in having parallel talks, talking about trade at the same time as divorce, for example . The position of the eu will be determined on the basis of careful analysis of theresa mays letter. There can be no future settlement if we are not clear on how the divorce settlement is going to be. To make two years of complex negotiations even thornier, the uk isnt talking just to the European Commission. The real power behind the throne lies in the eu capitals, berlin, paris, rome and 2a others. They will take any big political decisions for the eu when it comes to brexit and a Future Trade Deal. They dont and wont always agree with one another. And the article 50 time frame is very, very tight. Divisions there may be, but when it comes to the loaded issue of parallel trade and divorce talks, europes most powerful leader agrees with the European Commission. Much to demonstrates dismay. Translation in the negotiations we first have to sort out how we can untangle ourselves from one another. Only when that has been settled, hopefully soon after, we can speak about our future relationship. Chancellor merkel also stressed the importance of deciding the fate of eu citizens living in the uk, and british citizens in the eu, asap. Brussels and london agree. Thankfulfor one issue at least to unite around. Katya adler, bbc news, brussels. Lets assess the significance of today and what lies ahead. In a moment well speak to our europe editor katya adler in brussels, but first our Political EditorLaura Kuenssberg is with me. What have we learned today about what the next two years are going to bring . I think the tone of theresa mays letter which was, we love you, but were very sorry were leaving, tells us she is worried enough about the resistance she will face, she knows she has to butter them up. She had to set out a more conservatory tone that show shes aware this is going to be very difficult and also plenty of hints in their shoes where there will have to be give and take, compromise. Thats not what hes been the tone in the days and months since the referendum. It is also very clear shes not just since the referendum. It is also very clear shes notjust going to play nice. The fact theresa may gave theissue play nice. The fact theresa may gave the issue of security such prominence in the letter to other eu leaders today was notable. Shes made this point before, privately to eu leaders and publicly as well, but the way in which it was given such prominence in the letter has certainly been controversial, has raised eyebrows somewhere. One Cabinet Minister said to me, these are ourcards, Cabinet Minister said to me, these are our cards, these are our strengths and we shouldnt be surprised the Prime Minister is, in what will be a tough negotiation, willing to flex that particular muscle. Weve also seen today and early glimpse of the kind of resista nce early glimpse of the kind of resistance she knows shes going to face. W e will talk later, Thank Sta Kes as face. W e will talk later, thank stakes as we know are very high for the uk but this is not without straight to brussels. The stakes as we know are very high for the uk but this is not without risk for from. Absolutely. You saw the grim face of donald tusk, the president of the European Council as he received that letter from his heavy European Council as he received that letterfrom his heavy heart, the fa Ct Letterfrom his heavy heart, the fact this is such a huge moment for the eu, that was playing for everybody to beginning, there are no upside for the while in the uk there are many Who See Brexit as a cause of celebration, a chance for a new beginning, there are no upsides for the it is losing an influential member with a powerful economy, one of the to the eu budget and one two military mites in do so what will it is the big concern for the eu. While eu leaders defiantly say now through brexit they will unite even closer, the fact is this is an already weakened eu whose members fall out overfunding, over weakened eu whose members fall out over funding, over migration weakened eu whose members fall out overfunding, over migration and, without britain . That is the big concern for the eu. While eu leaders defiantly say now through brexit they will unite even closer, the fa ct they will unite even closer, the fact is this is an already weakened eu whose members fall out over funding, over migration the euro. So how can they stay united over something as complicated as brexit when each country has its own more likely to want to poland for example will want to safeguard the rights of its citizens who live in the uk, so its citizens who live in the uk, so it may be more likely to want to appease germany is putting politics ensure the integrity of the European Union and Single Market. It wants to ensure the integrity of the European Union and single has the upper there is one Ray Of Sunshine for the eu. It believes it has the upper hand because unlike the uk it is not under such time pressure to get a deal done. Thank you. The brexit clock started ticking from the moment the letter was delivered to brussels today, and the parties will have two years to conclude negotiations on britains exit. So what happens next . On friday, the European Council will set out its proposals for the negotiations to come. A month later at a Special Eu Summit the other 27 Member States will debate and then be asked to approve those proposals. The talking will then start, but Big Decisions may have to wait till after the french and german elections held in may and september. The negotiations should finish by october next year, when the uk and eu parliaments would both vote on the final deal. Thats the plan, but the path ahead is uncertain and there are no precedents for it. Our Diplomatic CorrespondentJames Robbins has been looking at the challenges ahead, as the negotiations get underway. The Europe Which Britain married into over a0 years ago looked and felt quite different. Just nine states in a predominantly economic community. Steadily membership grew, the ties that bind reached further and further across europe. The project became more political, the union ever deeper, until british voters opted for divorce. But that very complexity makes cutting the ties and agreeing th