Prime Minister Boris Johnson has been accused of showing little regard for the union after commenting that Scottish devolution has been "a disaster".
UUP leader Steve Aiken said the PM had "again been blundering about over the union".
"For someone who has claimed to cherish the United Kingdom he seems to take little care before he opens his mouth on the issue," Mr Aiken said.
The South Antrim MLA was speaking after reports that Mr Johnson said Scottish devolution had been a "disaster north of the border".
The Sun has reported that the PM said devolving powers was his predecessor Tony Blair's "biggest mistake" and he didn't see the reasoning for handing more powers to the Scottish Parliament.
Mr Johnson reportedly made the comments during a Zoom meeting with northern Conservative MPs on Monday.
Downing Street has not denied the remarks, with a source saying that the Prime Minister has "always supported devolution".
However, the source said that "Tony Blair failed to foresee the rise of separatists in Scotland".
"Devolution is great - but not when it's used by separatists and nationalists to break up the UK," the source said.
The SNP is currently pushing for a second Scottish independence referendum ahead of Scottish Parliament elections next May.
Mr Aiken said the PM had bigger issues on his plate than worrying about Scottish devolution.
"He could spend more time focusing on the real threat to the Union, which is the border the UK Government is allowing to be placed between Northern Ireland and Great Britain," the UUP leader said.
"We are weeks away from the end of the transition period and there has been little other than empty rhetoric from the Government on what they are going to do about it.
“Business and society are crying out for certainty; Boris Johnson needs to step up and sort it out.”
SDLP MLA and former Downing Street adviser Matthew O'Toole said Mr Johnson had offended the vast majority of people in Scotland.
"We're seeing a widespread contempt for devolution and a nascent English nationalism and I think that's whats underpinning a lot of what Boris is saying," the South Belfast MLA said.
"Last night he was talking to northern MPs and I think most people examining this honestly would link a lot of the rise of the Conservative Party in the north of England to the rise of English nationalism and it's clear that he was indulging quite a strong undercurrent of English nationalism within his own party."
In response to the PM's remarks, Scotland's First Minister Nicola Sturgeon suggested independence was the best answer for the devolved administration.
Ms Sturgeon 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 and strengthen the Scottish Parliament is with independence."
Communities Secretary Robert Jenrick said the break-up of the UK through separatism and nationalism would be a "dangerous and disappointing outcome".
The row comes only a month after the UK's three devolved administrations, including the Northern Ireland Assembly, raised concerns over the Westminster Government's UK Internal Market Bill, with the Scottish Government describing the legislation as a "power grab".