Scotland's Nicola Sturgeon has formally requested a second referendum on Scottish independence.

The first minister sent U.K. Prime Minister Theresa May a letter Friday formally requesting the transfer of powers to allow a second vote.

The letter followed a 69-59 vote in the Scottish Parliament this week in favour of asking for an independence referendum. The Scots want the vote to take place between autumn 2018 and spring 2019.

"In these very changed circumstances, the people of Scotland must have the right to choose our own future — in short, to exercise our right of self-determination," Sturgeon wrote.

Scotland's First Minister Nicola Sturgeon, left, said there appeared to be 'no rational reason' for U.K. Prime Minister Theresa May and her government to deny a second independence vote. (Russell Cheyne/PA via AP)

The British government has said it will deny the request.

May has made clear she does not believe it is the time for another vote. Scottish voters rejected independence in a 2014 referendum that Sturgeon's Scottish National Party called a once-in-a-generation vote. But Sturgeon says Brexit has altered conditions dramatically.

May's Downing Street office confirmed that the letter had been received and there would be a response in due course.

'The question is not if, but how?'

Sturgeon said there appeared to be "no rational reason" to deny a vote.

"It is my firm view that the mandate of the Scottish Parliament must be respected and progressed," she wrote. "The question is not if, but how?"

"I hope that will be by constructive discussion between our governments," Sturgeon continued. "However, if that is not yet possible, I will set out to the Scottish Parliament the steps I intend to take to ensure that progress is made towards a referendum."

Some 62 per cent of Scottish voters were in favour of remaining in the EU in last June's plebiscite.