Lord Mandelson has fired a warning shot at left-wing Labour MPs, warning they could be ousted if they fail to back Keir Starmer's manifesto.
The former New Labour Cabinet minister said MPs in the party's Socialist Campaign Group faction faced a choice before the next election, to get behind the party's offer or leave.
The SCG includes well-known figures who served in Jeremy Corbyn's shadow Cabinet, including John McDonnell, Richard Burgon, Ian Lavery and Dawn Butler.
Speaking at an online event with the Reform Scotland think tank, he said: "I am personally favour abroad church in our party, but no, if the Socialist Campaign Group, the hard left in the in the parliamentary party, if they want to be MPs, and they want to stand again, they have got to commit without equivocation to the democratically agreed manifesto of our party and its implementation by the next Labour government.
"No ifs, no buts. If they want to sign up to this journey and be members of the team, then they've got to agree that they will abide by the democratic decision-making of the party.
"There can't be standouts from the manifesto on which we are elected by the British people. That's fundamental."
Lord Mandelson, who was a close ally of Tony Blair's, is not thought to have an official role within Sir Keir's team, but reports suggest he is regularly in touch with senior figures.
He heaped praise on the Labour leader, who he said had shown "a great deal of personal grit" and "judgment, verve, dignity" in recent months and had "crossed a threshold in many people's minds" as a possible PM.
But he said Sir Keir and the shadow Cabinet had to show more strength within the party, adding: "If the British public get the idea that we're going to have a job-share of the top of our party between Corbyn and Starmer, then they'll not let us anywhere near the front door of No10."
He went on to say the left of the party had already been sidelined, adding: "There can be no going back.
"Momentum has now receded into the fringes or the undergrowth somewhere.
"We've routed out a lot of anti-Semites in the party, and that's got to continue. The National Executive Committee of our party is now in absolutely the historic mainstream of our party.
"Those are major, major achievements."
The former Business Secretary, who played a leading role in both the Remain campaign and the push for a second referendum on Brexit, also savaged the Prime Minister over his record in Government.
It comes as Boris Johnson is under increasing pressure to resign over rule-busting parties in Downing Street and his conduct as PM.
Lord Mandelson said: "Now, New Labour certainly had its arguments in government.
"But they were arguments of real substance, and they were conducted between real adults and they were resolved by adults. you don't really have that sense now about a government that seems to be sort of just ricocheting around the shopping trolley of a prime minister, without any clear sense of what they're doing, why they're doing it or where they go.
"He's not really a public servants at all. I mean, he is there not for what he can do but basically because he enjoys the attention, in that he's like Trump."
Asked what advice he would give the beleaguered PM as Labour soars in the polls, he added: "I didn't know what my advice would be I think probably: go for the sake of the country and stay for the sake of Labour."
He also lashed out at Jacob Rees-Mogg, the new Minister for Brexit Opportunities, calling him a "rather lethargic individual", adding that the UK must be "agile" and "identify [trading] opportunities, but don't make careless mistakes and and throw away our existing trade just for the ideological reasons".
The former first secretary of state went on to point out Labour had to develop forward-thinking policy IN reshaping economy as UK leaves single market, tech, skills and automation and achieving net zero.
