Nicola Sturgeon has called for a second independence referendum to take place within two years.

The First Minister said Scotland was at a "crossroads" as the UK Government prepared to trigger Article 50.

The earliest time for a vote would be the autumn of 2018 and the latest in the spring of 2019, she said.

She will now seek a Section 30 order to transfer power for the vote to the Scottish Parliament.

The First Minister indicated there was little chance of a compromise with the UK Government on the issue of Scotland having a special deal to stay in the European single market.

Theresa May could begin the formal process of leaving the EU as early as Tuesday, following a vote at Westminster.

The UK Government said "another referendum would be divisive and cause huge economic uncertainty."

Speaking at Bute House, First Minister said: "Scotland stands at a hugely important crossroads.

"On the eve of Article 50 being triggered, not only is there no UK-wide agreement on the way ahead, the UK Government has not moved even an inch in pursuit of compromise and agreement.

"All of our efforts at compromise have been met with a brick wall of intransigence."

She continued: "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.

"And far from any prospect of significant new powers for the Scottish Parliament, the UK Government is becoming ever more assertive in its intention to muscle in on the powers we already have.

"The language of partnership has gone, completely."

Sturgeon added: "The Scottish Government's mandate for offering this choice is beyond doubt.

"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.

"The UK Government was clear in 2014 that an independence referendum should be, in their words, 'made in Scotland, by the people of Scotland' - that is a principle that should be respected today.

"The detailed arrangements for a referendum - including its timing - should be for the Scottish Parliament to decide."

Sturgeon stressed there should be "clarity" about the implications of independence.

She said: "If I ruled out a referendum, I would be deciding - completely unilaterally - that Scotland will follow the UK to a hard Brexit come-what-may, no matter how damaging to our economy and our society it turns out to be."

Immediately following the Brexit vote last June, the First Minister said a second independence referendum was "highly likely".

Scots voted 62% to 38% to remain the EU while the UK as a whole voted to leave.

In the independence referendum in 2014, the vote was 55% to 45% in favour of staying in the UK.

The Scottish Parliament does not hold the power to organise an independence referendum and must gain permission from the UK Government to hold one through a legal instrument called a section 30 order.

The UK Government and several Scottish opposition parties have said there should not be a second vote on the issue.

Responding to the First Minister's comments, Prime Minister Theresa May said the majority of the Scottish people do not want a second independence referendum.

She said: "The tunnel vision that the SNP has shown today is deeply regrettable, it sets Scotland on a course for more uncertainty and division. Politics is not a game."

A UK Government spokesman said: "As the Prime Minister has set out, the UK Government seeks a future partnership with the EU that works for the whole of the United Kingdom.

"The UK Government will negotiate that agreement but we will do so taking into account the interests of all of the nations of the UK.

"We have been working closely with all the devolved administrations - listening to their proposals, and recognising the many areas of common ground, including workers' rights, the status of EU citizens living in the UK and our security from crime and terrorism."

He added: "Only a little over two years ago people in Scotland voted decisively to remain part of our United Kingdom in a referendum which the Scottish Government defined as a 'once in a generation' vote.

"The evidence clearly shows that a majority of people in Scotland do not want a second independence referendum.

"Another referendum would be divisive and cause huge economic uncertainty at the worst possible time. The Scottish Government should focus on delivering good government and public services for the people in Scotland."

Responding to the First Minister's speech, Scottish Labour leader Kezia Dugdale said "Scotland is already divided enough".

She said: "We do not want to be divided again but that is exactly what another independence referendum would do.

"Two years ago, 85% of Scotland's voters took part in the independence referendum and the result was a clear vote to remain in the UK.

"With our country facing all of the uncertainty around the Tories' reckless plans for a hard Brexit, the last thing we need is even more uncertainty and division."

She added: "A clear majority of the people of Scotland voted to reject the SNP's false hope and lies, and backed working together with the other nations of the UK.

"The reality is that leaving the UK would mean turbo-charged austerity for Scotland, putting the future of our schools and hospitals at risk."

Labour leader Jeremy Corbyn said he opposed a second referendum taking place but said the opposition should take place in the Scottish Parliament.

A spokesman said: "Jeremy and Labour do not want a second referendum but believe it would wrong for Westminster to block it."

Scottish Conservative leader Ruth Davidson said her party would oppose the move for a Section 30 order in the Scottish Parliament.

She tweeted: "Nicola Sturgeon has today chosen the path of further division and uncertainty."

Last week, a poll for STV News found Scots certain to vote in another independence referendum were split 50/50 on the issue.

A poll published in the Herald newspaper on Monday suggested the split was 52% in favour of staying in the UK and 48% for leaving.

The BMG research found 56% were against the idea of holding an independence vote before the Brexit negotiations are finished.