John McDonnell calls for complete Labour 'clean-out' over Mandelson row and says McSweeney 'controlled' No10

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Speaking to LBC's Matt Frei, the MP for Hayes and Harlington said next week's testimony from Olly Robbins would be crucial.

"I think it's absolutely critical," McDonnell said. "There's no rush to get rid of the Prime Minister because there's no appetite for that when you're in the middle of a war.

"But we're waiting to see what his response is on Monday, hear Olly Robbins and maybe others Tuesday and next week, and I think people will make a judgement then."

Robbins, the most senior civil servant at the Foreign Office, was sacked on Thursday night after it emerged the department cleared Mandelson to take up his Washington post despite concerns raised during the security vetting process.

Read More: Mandelson scandal is so serious it could 'certainly bring down' Prime Minister, Labour MP tells LBC

Read More: Olly Robbins has been ‘thrown under the bus’, says former Foreign Office chief

Starmer said he was "absolutely furious" at not being told.

"It's the McSweeney and Mandelson faction around Labour together that have dominated for six years," he said. "They dominate Number 10, even to the point of what is told to the Prime Minister."

McSweeney, who resigned as Starmer's chief of staff in February, is widely seen as the architect of the centrist network that has dominated the party and Number 10, with loyalists still in key positions across government.

He suggested Starmer himself may have been kept in the dark. "Mandelson wanted the job. He's got his people controlling number 10, McSweeney in particular, one of his proteges.

"And I think the civil servants got the message, he's going to get the job no matter what, get on with it," McDonnell told LBC.

"I wonder how much was told to Keir and how much they were maybe, using that phrase, economic with the truth, with him."

McDonnell acknowledged no obvious successor was waiting in the wings, but said changing leaders without changing the culture would solve nothing: "Replacing Keir Starmer with Wes Streeting or anyone else or Angela Rayner or whatever, but still having the same toxic culture — we're back here again."

He described Andy Burnham as the "obvious replacement who was emerging," but noted that he had been blocked from standing in a by-election.

"We need an absolute clean out," he said. "And I think if Keir can do anything, that's one of the things he should address."

On whether he would personally call for Starmer to go, McDonnell said he wanted "to know the truth" before making a decision.

He added: "I think he will take the responsibility into his own hands if he thinks he's done something wrong. I actually do think he'll have the responsibility on his shoulders to take that decision. But let's see what comes out of next week."

LBC has approached Morgan McSweeney for comment.