Scottish Labour leader Anas Sarwar is to call on Keir Starmer to resign as prime minister in a hammer blow for the prime minister.
He made the incendiary demand at an emergency press conference on Monday afternoon.
It comes as No.10 continues to deal with the fallout of the Peter Mandelson scandal.
The disgraced former UK ambassador to Washington is facing a police investigation over allegations he passed market sensitive information to the convicted paedophile Jeffrey Epstein when he was business secretary between 2008 and 2010.
The Scottish Labour leader’s ultimatum is a last-ditch effort to prevent annihilation for the party at the Holyrood elections on May 7.
Opinion polls show the SNP on course to be comfortably re-elected as the Scottish government, with Labour locked in a battle with Reform UK for second place.
Sarwar said: “The distraction needs to end and the leadership in Downing Street has to change.
“It is so obvious that we desperately need change in Scotland and in three months time the opportunity to get rid of a failing SNP government is one that is too important to be missed.
“We cannot allow the failures at the heart of Downing Street to mean the failures continue here in Scotland, because the election in May is not without consequence for the lives of Scots.”
No.10 will now be on red alert for any government ministers – especially those with Scottish seats – following Sarwar and demanding the PM’s resignation.
A Downing Street spokesperson said: “Keir Starmer is one of only four Labour leaders ever to have won a general election.
“He has a clear five-year mandate from the British people to deliver change, and that is what he will do.”
HuffPost UK has learned that even senior members of Sarwar’s frontbench team at the Scottish Parliament did not know he was going to call on the PM to quit.
One MP said: “The first rule of politics is don’t call for someone’s resignation unless you know they’re going to resign.
A Labour source said: “Let’s be clear, there is no way Anas makes this big and wrong call without sign off from Douglas Alexander, a member of Keir’s cabinet and co-chair of the Scottish Labour 2026 campaign, and Kate Watson, his chief of staff and a UK Labour employee.
“Both their positions must surely now be untenable.”
But Alexander said: “Anas has reached his own decision and I respect that. My job – as Anas acknowledged today – is to make sure Scotland’s voice is heard in cabinet. My focus remains on doing that.
“The prime minister has recognised not just that lessons have to be learned but also that we change how we do government. He is right about that and has my support.
“Like all Labour MPs we want the Labour government to be the government that the people of this country hoped for when they rejected the Tories. That is where all of my energies are and will remain focussed.”
Downing Street chief of staff Morgan McSweeney, Starmer’s closest adviser, dramatically quit on Sunday, saying he was taking responsibility for advising the PM to give Mandelson the ambassador’s job.
He was followed through the No.10 exit door on Monday morning by communications director Tim Allan.
Despite the chaos engulfing the No.10 operation, Starmer insisted the government was “going forward” as he addressed Downing Street staff just minutes before Allan’s resignation.
He said: “We must prove that politics can be a force for good. I believe it can. I believe it is. We go forward from here. We go with confidence as we continue changing the country.”
But a senior Labour source told HuffPost UK the PM was being “delusional”.