Scotland's first minister has signed a letter formally asking for powers to hold a second Scottish independence referendum.
The Scottish Parliament voted by 69 to 59 on Tuesday in favour of seeking permission for another referendum.
Nicola Sturgeon argues Scotland should have the choice on what path to follow in the wake of the Brexit vote.
The UK government has already indicated that any referendum should wait until the Brexit process has been completed.
Prime Minister Theresa May, who met Ms Sturgeon for talks in Glasgow on Monday, has repeatedly insisted that "now is not the time".
She argues that the focus should be on getting the best Brexit deal for the whole of the UK, and Scottish voters can only make an informed choice once the terms are clear.
By Sarah Smith, Scotland editor
Sitting on the sofa, her shoes kicked off, putting the final touches to that letter
It couldn't be more different from the very formal portrait of Theresa May signing the Article 50 letter in Downing Street. Very different images - and that is no accident.
But while the picture may look quite casual, the contents of this letter are not.
In it the first minister asserts that she has a clear mandate to ask for another referendum since the Scottish parliament voted to back her on Tuesday.
And she repeats her request for a vote in 18-24 months time. She says by then the shape of the Brexit deal will be clear.
But she knows what the prime minister is going to say in reply.
Constitutional matters are reserved to Westminster so the Scottish government must ask for the powers to hold such a vote to be transferred to Holyrood under a Section 30 order, as was done before the 2014 referendum.
The Scottish government released a photo of Nicola Sturgeon drafting the letter to Theresa May, with her feet curled up on a sofa at her official residence, Bute House in Edinburgh.
The letter is expected to be sent to Downing Street on Friday.
Ms Sturgeon is seeking a referendum between the autumn next year and spring 2019 - but has indicated she would be willing to negotiate the timing.
If, as expected, the request is declined, Ms Sturgeon has said she will set out her government's next steps in April, when MSPs return to the Scottish Parliament after the Easter recess.
The two-day debate at Holyrood on an independence referendum began last week but was suspended as news of the terror attack at Westminster emerged.
When it resumed on Tuesday, the minority SNP government was backed by the pro-independence Scottish Greens in the vote, with the Conservatives, Labour and Lib Dems opposed.
Scottish voters rejected independence by 55% to 45% in a referendum in 2014, but Ms Sturgeon believes the UK voting to leave the EU is a material change in circumstances which means people should again be asked the question.
While the UK as a whole voted to leave, Scotland voted by 62% to 38% to remain in the EU.