Nicola Sturgeon today demanded that Holyrood be given the power to hold a second independence referendum between autumn 2018 and spring 2019.
The first minister said Scottish voters must be given the choice of whether to "follow the UK to a hard Brexit or to become an independent country".
She will next week ask Theresa May for a so-called "Section 30 order" to ensure the result of any vote is legally-binding.
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Sturgeon made her dramatic announcement at a hastily-arranged press conference at her official residence in Bute House, Edinburgh. She said:
"I will continue to stand up for Scotland's interests during the process of Brexit negotiations.
"I will now take the steps necessary to make sure that Scotland will have a choice at the end of this process, 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.
"The Scottish government's mandate for offering this choice is beyond doubt. Last year we were elected with the highest share of the constituency vote own by any party in the history of devolution on a manifesto that said this – 'the Scottish Parliament should have the right to hold another referendum if there is a significant and material change in the circumstances that prevailed in 2014, such as Scotland being taken out of the EU against our will'.
"These conditions have, of course, now been met so I can confirm today that next week I will seek the authority of the Scottish parliament to agree with the UK government the details of a section 30 order – the procedure that will enable the Scottish parliament to legislate for an independence referendum.
"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."
It means Scots could go to the polls again just four years after they voted to remain part of the UK in the last referendum.
The SNP leader also launched a withering attack on Theresa May, accusing the prime minister of "intransigence" and a refusal to compromise over the Scottish government's Brexit demands.
"Over the past few months we have worked hard, really hard, to try and find agreement, the Prime Minister and her government have been given every opportunity to compromise," she said.
"But today as we stand, for all we know, on the eve of Article 50 being triggered, not only is there no UK-wide agreement on the way ahead but the UK government has not moved even an inch in pursuit of compromise and agreement. Our efforts at compromise have instead been met with a brick wall of intransigence."
She said Brexit had meant that the “option of no change is no longer available”, but that it was now for the Scottish people to “decide the nature of that change”.
The SNP leader added: “That should not be the decision of just one politician, not even the First Minister. By taking the steps I have set out today I am ensuring that Scotland’s future will be decided not just by me, the Scottish Government or the SNP; it will be decided by the people of Scotland.
“It will be Scotland’s choice and I trust the people to make that choice.”
Speaking before Sturgeon's announcement, a spokesman for the prime minister insisted the government "will engage with the devolved administration throughout this process".
The first minister's intervention comes just hours before parliament is expected to pass the Article 50 bill, allowing Theresa May to start the Brexit process as early as tomorrow.