Boris Johnson has formally rejected a call for Holyrood to be given the power to hold a second independence referendum.
The British PM said holding such a vote would mean “political stagnation” for Scotland.
He said it was instead time for the the nations to “work together and unleash the potential of this great country”.
Scots voted by 55% to 45% to stay in the United Kingdom in a referendum in 2014.
In his letter to Ms Sturgeon, Mr Johnson said he had “carefully considered” the arguments she made.
But he said Ms Sturgeon and her predecessor Alex Salmond had made a “personal commitment” that the 2014 referendum would be a “once in a generation” event.
“The UK Government will continue to uphold the democratic decision of the Scottish people and the promise that you made to them,” he said.
“For that reason I cannot agree to any request for a transfer of power that would lead to further independence referendums.”
Mr Johnson added: “Another independence referendum would continue the political stagnation that Scotland has seen for the last decade, with Scottish schools, hospitals and jobs again left behind because of a campaign to separate the UK.
“It is time that we all worked to bring the whole of the United Kingdom together and unleash the potential of this great country.”