On Monday Ms Sturgeon set out a plan for a vote on Scottish independence some time after autumn 2018, when she said the terms of the Brexit deal would be clear, but before the anticipated exit from the European Union in the spring of 2019.

The new vote - already dubbed 'indyref2' - would come just four years after nationalists lost the previous independence referendum, when 55 per cent of people voted to remain in the UK.

In 2014 Ms Sturgeon's predecessor Alex Salmond pledged the vote would be a "once in a generation, perhaps even a once in a lifetime" opportunity. Ms Sturgeon, then his deputy, echoed the line.

However Ms Sturgeon argued that the Brexit referendum last year had changed the political equation, and Scotland stood "at hugely important crossroads".

"In 2014, we didn't know the UK would vote to leave the EU - had we done so it is likely that some, perhaps on both sides, would have come to a different decision," she said in a speech on Monday at Bute House, the first minister's official residence in Edinburgh.

"The future of the UK looks very different today than it did two years ago.

"As a result of the Brexit vote we face a future, not just outside the EU, but also outside the world's biggest single market.

"In addition, the collapse of the Labour Party means that we face a prolonged period of uninterrupted and unchecked Conservative government at Westminster.

"Some predict that the Tories could be in power now at Westminster until 2030 or beyond."

She said Brexit would see powers return to Westminster which were currently wholly devolved to the Scottish parliament.

Ms Sturgeon said she had tried to explore a way Scotland could retain access to the European single market despite Brexit.

However, she said, "our efforts at compromise have instead been met with a brick wall of intransigence.

"UK membership of the single market was ruled out with no prior consultation with the Scottish Government or with the other devolved administrations - leaving us facing not just Brexit, but a hard Brexit.

"There has been talk of special deals for the car industry and others, but a point blank refusal to discuss in any meaningful way a differential approach for Scotland."

Next week Ms Sturgeon will seek approval from the Scottish parliament - dominated by the nationalists - for her plan.

The British government will then have to agree to a new referendum.

Prime Minister May told the BBC the majority of the Scottish people didn't want a second referendum, and accused the SNP of "tunnel vision" and "playing politics".

"It sets Scotland on a course for more uncertainty and division," she said.

Recent polls in Scotland show a majority are still against independence should there be another vote - but by a narrower margin than the end result in the 2014 referendum - and in the last year some polls showed a majority would vote 'yes'.

In last year's Brexit referendum Scotland voted 62 per cent to 38 per cent to Remain in the European Union, and polls show that more than a quarter of Scots who voted against independence in 2014 voted for Remain in 2016. It is these voters that the nationalists would seek to court for Indyref 2.

Scottish Conservative leader Ruth Davidson said Ms Sturgeon's proposal "offers Scotland the worst of all worlds" and was "utterly irresponsible".

"Both No and Yes voters have been urging her to put this to one side - but because of her own rash decision to use Brexit in a bid to lever support for independence, she has ignored them completely," she said.

Labour leader Jeremy Corbyn said his party opposed Scottish independence, and will oppose the vote in the Scottish parliament. However if the vote passed in Holyrood, Labour would not try to block a second referendum from Westminster.

Scottish Labour leader Kezia Dugdale said leaving the UK "would mean turbo-charged austerity for Scotland".

Scottish Liberal Democrat leader Willie Rennie said the Scottish National Party had "been determined to contrive a way to ignore their promise that 2014 was once in a generation".

"There is no wide public support for a new and divisive referendum," he said, saying the SNP's policy risked leaving Scotland outside both the UK and the EU, as it would not automatically 'inherit' the UK's EU membership.

In response to the announcement, on Twitter a Nicola Sturgeon 'parody account' posted a video compiling all the times Ms Sturgeon had previously described the 2014 referendum as a "once in a generation" and "once in a lifetime" opportunity for Scotland.