By Elisabeth O'Leary
EDINBURGH (Reuters) - Scotland's First Minister Nicola Sturgeon on Monday demanded a new independence referendum in late 2018 or early 2019, once the terms of the United Kingdom's exit from the European Union have become clearer.
An independence vote that could rip apart the world's fifth largest economy just months before Brexit raises the stakes for Prime Minister Theresa May as she prepares to trigger two years of formal Brexit talks.
"If Scotland is to have a real choice - when the terms of Brexit are known but before it is too late to choose our own course - then that choice must be offered between the autumn of next year, 2018, and the spring of 2019," Sturgeon told reporters in Edinburgh.
In the June 23 Brexit referendum, the UK-wide vote was 51.9 - 48.1 percent to leave the EU. Voters in England and Wales chose to leave while in Scotland and Northern Ireland they voted to stay, and since then the strains on the United Kingdom have deepened.
Hours after Sturgeon spoke, Northern Ireland's largest Irish nationalist party Sinn Fein said it wanted a referendum on splitting from the United Kingdom "as soon as possible".
"Brexit will be a disaster for the economy, and a disaster for the people of Ireland," Sinn Fein's leader in Northern Ireland Michelle O'Neill told reporters in Belfast. "A referendum on Irish unity has to happen as soon as possible."
Sturgeon, who heads Edinburgh's pro-independence devolved government, said she would next week seek the approval of the Scottish Parliament to discuss with the UK government the details enabling a second referendum to take place.
The detailed arrangements for a referendum - including its timing - should be decided by Scotland's parliament, she said.
Scots rejected independence by 55-45 percent in a referendum in September 2014, but support for Sturgeon's Scottish National Party has surged since then.
It is ultimately the UK parliament in Westminster - where May commands a majority - which decides whether Scotland can hold a second referendum.
But if she refused to approve such a vote she could provoke a constitutional crisis and stoke discord in Scotland.
While Sturgeon said the "door was still open" to talking to London, she added she was not expecting a change of tack over Brexit by May's government, which says it will prioritise immigration controls over continued preferential access to the European single market.
"I cannot pretend that a compromise looks remotely likely given the hardline response so far," she said.
May has not publicly said whether she would try to block a Scottish attempt to hold a second referendum, but has accused Sturgeon's Scottish National Party of sacrificing Scotland's interests through its "obsession" with securing independence.
"The tunnel vision that SNP has shown today is deeply regrettable," May said.
"Instead of playing politics with the future of our country, the Scottish government should focus on delivering good government and public services for the people of Scotland. Politics is not a game."
May's spokesman said the evidence showed that the majority of people in Scotland did not want a second independence referendum, which he said would be divisive and cause huge economic uncertainty.
Sturgeon has said she wants Scotland to have its own deal as part of the United Kingdom's Brexit agreement, to keep its preferential access to the single market.
But on Monday she said her efforts had hit a "brick wall of intransigence" in London and that she feared a bad deal or even no proper deal for the UK to leave the EU.
"I will now take the steps necessary to make sure that Scotland will have a choice at the end of this process," she said.
"A choice of whether to follow the UK to a hard Brexit - or to become an independent country, able to secure a real partnership of equals with the rest of the UK and our own relationship with Europe."
An independent Scotland born after a messy secession battle with London would have to apply for EU membership after the United Kingdom left, according to diplomats and EU officials.
Sterling gained broadly against major currencies after Sturgeon's announcement.
Recent opinion polls have shown support for independence rising since May announced Britain would quit the single market. A poll last week saw a 50-50 split.
Asked if she believed she could win a second independence vote, Sturgeon replied: "Yes I do. Absolutely, I believe that."
(Additional reporting by Andy Bruce in London; Writing by William Schomberg and Guy Faulconbridge; Editing by Andrew Roche)