By Elisabeth O'Leary
EDINBURGH (Reuters) - Scotland could hold an independence referendum in the autumn of 2018, just months before Britain is due to leave the European Union, Scottish First Minister Nicola Sturgeon told the BBC, and a second poll in a month suggested rising support for secession.
The prospect of an independence vote in Scotland that could rip apart the United Kingdom just months before an EU exit adds to the complexity of negotiations on Britain's departure from the European Union.
Scotland's threat of a second independence vote in four years also raises the pressure on Prime Minister Theresa May as she prepares to trigger formal exit negotiations with the other 27 members of the European Union over the UK's divorce terms.
Sturgeon said autumn 2018 would be a "common sense time" for Scotland to hold another independence referendum, once there is some outline of a deal to exit the European Union.
"Within that window, of when the outline of a UK deal becomes clear and the UK exiting the EU, I think would be a common sense time for Scotland to have that choice, if that is the road we choose to go down," Sturgeon, who heads Edinburgh's pro-independence devolved government, told the BBC.
No decision has yet been taken on a vote, she added.
Up to now, most polls show support for independence in Scotland has barely shifted from around 45 percent since 2014, and that most Scots do not want another vote on secession.
However, a poll by Ipsos MORI on Thursday showed that among those likely to vote, support for independence had risen to 50 percent, a 2 percent increase in support for independence compared to Ipsos Mori's last poll. They surveyed 1,029 people by telephone between Feb. 24 and March 6.
On Feb. 8, a BMG survey for Herald Scotland indicated 49 percent of Scots backed independence, up from 45.5 percent a month earlier.
Under the UK's constitutional conventions, an independence vote would have to be approved by May's government which on Thursday repeated it saw no need for a second ballot.
The results of the June 23 Brexit referendum called the future of the UK into question because England and Wales voted to leave the EU but Scotland and Northern Ireland voted to stay.
CLASH OVER BREXIT
Sturgeon has said that the Brexit plans of the government in London -- in particular May's decision to prioritise immigration controls over continued preferential access to the single market -- made another vote on independence necessary because circumstances have changed since 2014, when Scots voted 55-45 to stay in the United Kingdom.
The EU's chief negotiator for Brexit, Michel Barnier, has said that an exit agreement with the UK should be reached by October 2018 though many diplomats and trade negotiators have expressed concern that a comprehensive Brexit deal would be difficult to strike in such a short time.
Wary of secessionists in other EU states, the Union was cool to Scottish pleas to stay in the bloc during the independence referendum in 2014.
Brexit would be an added complication and an independent Scotland born through a messy secession battle with London would have to apply for EU membership after the United Kingdom left, according to diplomats and EU officials.
Spain, which faces secessionists in its northeastern region of Catalonia, may have little appetite to approve an independent Scotland's application for membership of the EU, which would have to be unanimously approved by EU members states.
May is due to trigger formal exit talks by the end of this month though legislation giving her approval to do so is unlikely to clear parliament until mid-March.
Sources close to Sturgeon say May's official notification of withdrawal from the EU is a key milestone, and no decision on a referendum would be made before she triggered formal talks.
"Common sense would tell you that ... if we are going to become independent, better to do so before we are dragged out of the EU," Stewart Hosie, an SNP lawmaker said.
Scotland has a population of around 5.3 million, according to the last census, slightly more than 8 percent of the United Kingdom's population as a whole. It was an independent kingdom until joining England in the Act of Union in 1707.
(Additional reporting by Alistair Smout and Alastair Macdonald; Editing by Guy Faulconbridge and Toby Chopra)