First Minister Nicola Sturgeon has said she will seek approval next week for a second referendum on Scottish independence.
She said a new vote should be held between autumn 2018 and spring 2019.
However, Sky sources say Ms Sturgeon had not been convinced about calling for the referendum so soon, fearing it is not yet winnable.
Former first minister Alex Salmond and his supporters are said to have pressured her to act.
Ms Sturgeon, making the announcement in Edinburgh, said she believed people would now choose to break away from the UK in order to stay in the EU.
She said it was about protecting Scotland’s interests in “circumstances we didn’t ask to be in”.
The First Minister wanted to arrange a special deal for Scotland to stay in the single market, but told reporters the UK government had “not moved even an inch” and refused to discuss “in any meaningful way a differential approach”.
Ms Sturgeon said Scotland was now at a “hugely important crossroads”.
Prime Minister Theresa May accused the SNP of “tunnel vision” and insisted she wants to negotiate a Brexit deal “for the whole of the United Kingdom”.
Ms Sturgeon said the proposed timing of the referendum would mean the terms of Brexit would likely be known and therefore voters could make an informed decision.
MSPs at the Holyrood parliament, where there is a pro-independence majority, must first back her call.
The proposal will then move to Westminster, where MPs and the Lords must also support the proposal.
“The Scottish Government’s mandate for offering this choice is beyond doubt,” said Ms Sturgeon.
“So next week I will seek the approval of the Scottish Parliament to open discussions with the UK Government on the details of a Section 30 order – the procedure that will enable the Scottish Parliament to legislate for an independence referendum.”
In 2014, Scotland voted to stay part of the UK by 55.3% to 44.7%; but in the Brexit vote Scots voted to remain in the EU by 62% to 38%.
Mrs May was quick to react to Ms Sturgeon’s announcement, hitting back at her claim that the Government had put up a “brick wall” over Brexit arrangements.
“We’ve been working closely with the devolved administrations,” said the PM.
“We’ve been listening to their proposals; and recognising the many areas of common ground we have, such as protecting workers’ rights and our security from crime and terrorism.”
Mrs May added: “The tunnel vision that the SNP has shown today is deeply regrettable.
“It sets Scotland on a course for more uncertainty and division,” she said.
“This is at a time when the evidence is that the majority of the Scottish people do not want a second independence referendum.
“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.”
Labour leader Jeremy Corbyn said it was “wrong to hold another (referendum) so soon” and that the party opposed independence.
However, he added that Labour would not block a second referendum if the Scottish Parliament votes for it.
Scottish Conservative leader Ruth Davidson tweeted that Ms Sturgeon had “chosen the path of further division and uncertainty”, while Lib Dem leader Tim Farron also said he opposed another vote.
Mrs Sturgeon’s announcement came as Mrs May is expected to trigger Article 50 this week and begin the formal start of Brexit negotiations.
