BBC News
By Glenn Campbell
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The only way for there to be a successful referendum on Scottish independence is if both sides agree.
That's how they did it in 2014 with the Edinburgh agreement and the alternatives are messy and risky for both sides.
If the current stand-off between Nicola Sturgeon and Boris Johnson cannot be resolved, the first minister intends - at some point - to pass a referendum bill at Holyrood.
She dares the UK government to challenge the legislation in court which it may feel obliged to do because it considers such a vote to be beyond devolved powers.