A letter formally requesting a second Scottish independence referendum is to arrive at Downing Street on Friday.
The section 30 letter was signed by First Minister Nicola Sturgeon on Thursday.
This week, MSPs voted by 69 to 59 in favour of seeking permission for an independence referendum to take place between autumn 2018 and spring 2019.
However, Scottish Secretary David Mundell has already said the UK Government would decline the request.
Yet Ms Sturgeon is pressing ahead with a formal approach for a section 30 order - the mechanism for the powers to hold a referendum.
She said her mandate for another vote is now "beyond question", and warned it would be "democratically indefensible and utterly unsustainable" to attempt to stand in the way.
The contents of the letter will be published on Friday.
Around 62% of Scottish voters voted for the UK to remain in the EU in the June referendum and the SNP manifesto for the 2016 Holyrood elections made clear another ballot on independence should take place if there were a ''material change in circumstances'' from the previous ballot in 2014.
The example cited was for Scotland to be removed from the EU against its wishes.
Scottish Conservative, Labour and Liberal Democrat politicians oppose another referendum and the Prime Minister has repeatedly said ''now is not the time'' for another vote, indicating she will reject the SNP's preferred timetable.
After the vote in the Scottish Parliament, Ms Sturgeon said she would set out her next steps to Holyrood after the Easter recess if she is rebuffed in the Section 30 request.
The vote followed a meeting between Ms Sturgeon and Mrs May in Glasgow on Monday, and came the day before the process for leaving the European Union was formally triggered.