Former prime minister Gordon Brown and a band of regional English mayors have written to Boris Johnson to urge him to “save” the Union by handing out greater powers to its nations.
The politicians have warned the Prime Minister that “time is fast running out” to keep the UK intact and argued that the way to protect it was to create a more federal Britain.
Mr Brown said the letter, coordinated by Sheffield City Region mayor Dan Jarvis and backed by his London counterpart Sadiq Khan, Greater Manchester’s Andy Burnham and Liverpool’s Steve Rotheram, should serve as a “wake-up call for us all”.
Scotland voted to remain part of the UK at the 2014 referendum but First Minister Nicola Sturgeon, leader of the SNP, is pressing for a second independence vote.
The SNP has argued that a majority at the Holyrood elections next year would give them a mandate for round two.
It comes after the Prime Minister, who opposes a second referendum, was said to have told Tory MPs last month that devolution to Scotland had been a “disaster”.
In the letter, the politicians said: “The United Kingdom is in great danger.
“Recent polls show a clear trend towards Scots losing faith in the Union and a majority of Britons believe the UK is heading for break-up.
“That should be a clamorous wake-up call for us to examine the status quo.
“We believe the aim should be a more federal Britain of nations and regions, a free and equal partnership with stronger devolution bringing power closer to the people, especially in England.
“We have the chance not just to preserve our Union, but to create the extraordinary legacy of a more harmonious, united and democratic Britain for us all.
“Time is fast running out. We can still save our country but we need to act now.”
Ex-Labour leader Mr Brown said: “This intervention shows that the appetite for change and reform cuts across the UK. There is momentum and interest to build an alternative to the status quo.
“This letter is intended as a wake-up call for us all. It’s a call for a national conversation.
“It’s a call, rather than break our country apart, for us to try to solve our differences.”
The SNP’s Pete Wishart told Mr Johnson at Prime Minister’s Questions on Wednesday that he was “probably the biggest single recruiting sergeant that we have” following his decision to lead the Brexit vote and recent comments on devolution.