British Prime Minister Boris Johnson wrote to Scottish First Minister Nicola Sturgeon on Tuesday, refusing her request to be given the powers to hold another Scottish independence referendum.

As things stand, a referendum cannot take place without the consent of the U.K. government. Sturgeon wrote to Johnson in December asking him to enter negotiations on transferring the power to hold a referendum from London to Edinburgh.

"I cannot agree to any request for a transfer of power that would lead to further independence referendums," Johnson wrote in a letter which he posted on Twitter. He said he had told Sturgeon she had agreed that a 2014 referendum, in which Scots voted by 55%-45% to stay in the United Kingdom, would be a "once in a generation" vote. He added: "Another independence referendum would continue the political stagnation that Scotland has seen for the last decade... it is time that we all worked to bring the whole of the United Kingdom together."

Sturgeon argues that the 2016 vote to leave the European Union, with Britain set to leave the bloc on Jan. 31, warrants a new independence referendum because Scots overwhelming voted against Brexit while a majority of English voters supported it.

Polls suggest that Scots would narrowly reject independence again although Sturgeon's Scottish National Party won 48 of Scotland's 59 seats in Britain's national election last month, taking 45% of votes cast, an 8 percentage-point increase from 2017.

The Scottish first minister said Johnson's response to her request was predictable and that he was blocking another vote because he did not have a positive case for keeping the more than 300-year-old union. "While today's response is not surprising – indeed we anticipated it – it will not stand," Sturgeon said in a statement.

"It is not politically sustainable for any Westminster government to stand in the way of the right of the people of Scotland to decide their own future and to seek to block the clear democratic mandate for an independence referendum." She said the Scottish government would set out its next steps later this month and would seek the devolved Scottish parliament's backing again for another plebiscite. "Democracy will prevail," she added.