A row has erupted after Boris Johnson reportedly condemned devolution in Scotland, saying it had been "a disaster north of the border".
The prime minister claimed it was Tony Blair's "biggest mistake" and that while he previously thought it was a good idea, he could not "see a case" for handing down more powers to Holyrood, according to The Sun.
Downing Street did not deny the remarks were made at a virtual meeting with northern Tory MPs, and a Number 10 source stepped in to clarify them.
But that did not stop a furious reaction outside Westminster.
Douglas Ross, the Scottish Conservatives leader, flatly disagreed with the assessment, tweeting: "Devolution has not been a disaster.
"The SNP's non-stop obsession with another referendum - above jobs, schools and everything else - has been a disaster."
Nicola Sturgeon, Scotland's first minister, is likely to use the comments as ammunition ahead of next year's Holyrood election, where - if the SNP wins a majority - she will press for another independence referendum.
She tweeted: "Worth bookmarking these PM comments for the next time Tories say they're not a threat to the powers of the Scottish Parliament - or, even more incredibly, that they support devolving more powers.
"The only way to protect & strengthen the Scottish Parliament is with independence."
The timing of Mr Johnson's comments are particularly sensitive because the UK government is trying to pass its controversial Internal Market Bill.
The draft legislation, which apart from planning to break international law by overriding the Brexit deal, has also sparked concern Westminster will take back some powers from Brussels following Brexit that the SNP say should be devolved.
Ian Murray, Labour's shadow Scotland secretary, said: "The Tories have always been a bigger threat to the UK than any nationalist. Johnson with bells on."
Attempting to quell the backlash, a Number 10 source said Mr Johnson "always supported devolution", but that Mr Blair, under whom the Scottish Parliament was set up, "failed to foresee the rise of separatists".
"Devolution is great - but not when it's used by separatists and nationalists to break up the UK," the source said.
Cabinet minister Robert Jenrick told Sky News' Kay Burley that the prime minister "has always supported devolution" but is "at heart a unionist".
"He is very troubled by the rise of nationalism and separatism in the form of Nicola Sturgeon and the SNP," the housing secretary added.
"That, I think, is the comment he was making yesterday - that whilst in some parts of the UK devolution has enabled local people to have greater say over their own destinies, to make different and the right decisions for their own communities.
"One of the downsides in Scotland has been that it's been misused by the SNP to drive a wedge between people who are ultimately part of the same country with hundreds of years of history, of friendship and partnership.
"And the PM is very concerned by that."
'I've never thought he cares about Scotland': PM's comments push voters towards independence
By Dan Whitehead, Sky News correspondent
"It's nothing to do with him - I've never thought he cared about Scotland," said workman Peter Anderson, as he dug up the pavement outside the Scottish parliament in Edinburgh on Tuesday.
"Boris does not live up here, we should govern ourselves. It'll fuel Nicola Sturgeon's fire.
"I'm a strong supporter of independence - I voted for it. It makes me angry. He judges things he doesn't understand.
"I've never thought he cares about Scotland. England and London is all he cares about."
Kirsten Griew voted for Scotland to remain part of the UK, but said comments like those the prime minister was reported to have made caused her to think again: "Anytime you hear these comments it pushes me towards independence.
"I think the only thing Boris cares about is his own power and all his decision are made on that basis.
"I haven't supported independence - I voted against it last time but the way things have fermenting over the last few years it does push me closer."