May has the power to block the vote and said that another referendum, after Scots voted by 55 per cent to reject independence in 2014, would only cause "uncertainty and division".

But Sturgeon's call pushes to centre stage one of the prime minister's biggest concerns about Brexit - that it could lead to the break-up of the United Kingdom - as she prepares to fire the starting gun.

The other 27 European leaders are prepared for Britain to trigger Article 50, which begins a two-year countdown to Brexit, this week.

However, speculation is growing that it may now be delayed until after a Mar 25 summit in Rome to mark the EU's 60th birthday - timing that would likely be welcomed in Brussels.

Once May has notified the EU of her decision by letter, the bloc will take just 48 hours to issue its first draft proposal for the negotiations, with a follow-up meeting planned on Apr 6.

The actual talks are not expected to begin for months.