Boris Johnson has formally rejected a call for Holyrood to be given the power to hold a second independence referendum.
The 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, the PM 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.”
Last month, Scottish First Minister Nicola Sturgeon requested powers be transferred, saying there was a “democratic case” for a second ballot to be held on the issue.
Ms Sturgeon said on Tuesday: “Tories are terrified of Scotland’s right to choose – because they know that when given the choice we’ll choose independence.”
She added: “Tories have no positive case for the union – so all they can do is attempt to deny democracy. It will not stand.”
Responding to Mr Johnson via Twitter, the SNP leader said: “The problem for the Tories is the longer they try to block democracy, the more they show the Westminster union is not one of equals and fuel support for independence.
“This response (is) predictable – but also unsustainable and self defeating. Scotland will have the right to choose.”
She said the Scottish Government would set out its response and the next steps it will take before the end of January – adding she would also ask MSPs at Holyrood to “back Scotland’s right to choose our own future”.