Last year the City of London exported £25billion of services to the EU, making it a key market.
Sam Lowe, a trade expert at the Centre for European Reform who was previously also a member of the Government's Strategic Trade Advisory Group, has warned negotiations on financial services "could create political tension in the EU-UK relationship".
He said Brussels is yet to decide on the issue of so-called "equivalence" - determining if the UK's rules and regulations are suitably compatible with its own that firms in London can have access to the EU market.
He wrote in a blog: "The EU still needs to decide whether the UK’s financial services rule book is equivalent to its own, and if so whether to allow certain financial products to continue being sold into the EU from the UK’s territory.