Cabinet ministers have been rallying support for Sir Keir Starmer, ahead of a vote by MPs on whether he should face a parliamentary investigation over his claims about the vetting of Lord Mandelson.
The prime minister has denied accusations he misled MPs over whether the vetting for the role US ambassador followed "due process" and over his assertion that "no pressure whatsoever" was applied to officials at the Foreign Office.
Giving an impassioned speech to Labour MPs on Monday evening, the PM branded the vote, which was requested by the Conservatives, "pure politics" and urged them to "stand together against it".
The BBC has been told Labour MPs have been ordered to vote down the proposal.
It is unlikely the vote will pass as this would require a mass rebellion by Labour MPs and there is no evidence one is brewing.
However, it will take place after two former senior government figures - the PM's ex-chief of staff Morgan McSweeney and the former top civil servant at the Foreign Office Sir Philip Barton - give evidence to MPs on the Foreign Affairs Committee.
It is possible their accounts could change the mood among MPs.
In a sign of how politically damaging a major rebellion could be, there was a concerted operation by No 10 on Monday evening to ensure Labour MPs were on side.
This included cabinet ministers ringing round backbenchers and senior Labour figures, including former Prime Minister Gordon Brown, issuing public statements backing the prime minister.
One MP told the BBC they gave a slightly equivocal answer to their whip - responsible for party discipline - about how they would vote and got a phone call from a cabinet minister minutes later.
The motion which MPs will vote on was spearheaded by Conservative leader Kemi Badenoch but was also signed by Liberal Democrat leader Sir Ed Davey, as well as SNP, DUP and independent MPs.
It identifies three possible areas where the PM may have misled the Commons.
These include Sir Keir's repeated insistence that "full due process" was followed in Lord Mandelson's appointment, that he was "subject to developed vetting", that "nobody put pressure" on the top civil servant at the Foreign Office to make the appointment and that "no pressure whatsoever exists in relation to this case".
If the motion passes, the issue would be looked at by MPs on the Privileges Committee.
The cross-party committee can look into cases of MPs breaking parliamentary rules and in 2023 it ruled that the former Prime Minister Boris Johnson had misled MPs about parties in Downing Street during Covid.
The Ministerial Code states that ministers who knowingly mislead Parliament are expected to resign, while any inadvertent error should be corrected "at the earliest opportunity".
Badenoch has argued Sir Keir misled Parliament "multiple times" and urged Labour MPs to "look into their consciences" and back an inquiry by the Privileges Committee.
A Downing Street spokesperson said the claims "have no substance" and the government is "engaging with the two parliamentary processes that are already running" on Lord Mandelson's appointment "with full transparency".
On Monday, the government published a letter from September 2025 in which the then-head of the Civil Service Sir Chris Wormald told the prime minister that "appropriate processes" were followed in the appointment.
Written evidence from the Foreign Office done in consultation with Ian Collard, the civil servant who was head of security in the department, was also published on Monday by the Foreign Affairs Committee.
It noted that Collard "felt pressure to deliver a rapid outcome" on Lord Mandelson's vetting clearance due to "regular contact from No 10" with the department.
It added Collard "did not personally speak to colleagues in No 10" and he "does not assess that this pressure influenced the professional judgement that was reached by himself or his team".
This backed up the account of Sir Olly Robbins, who was the senior civil servant in the Foreign Office until he was sacked by the PM.
He told the Foreign Affairs Committee last week that there was "constant pressure" over when the vetting process would be completed, although he insisted this did not affect his decision to give Lord Mandelson security clearance.
In an apparent effort to clarify his comments in Parliament, Sir Keir told the Sunday Times there are "different types of pressure".
"There's pressure – 'Can we get this done quickly?' – which is not an unusual pressure. That is the everyday pressure of government," he said.
Liberal Democrat Cabinet Office spokeswoman Lisa Smart MP said Labour MPs "must put principle before party and vote to refer Keir Starmer to the Privileges Committee".
Reform UK leader Nigel Farage said he believed the prime minister had misled the House of Commons more than once.
"Boris Johnson tried it - didn't get away with it - no reason why Keir Starmer should," he added.
Zack Polanski, leader of the Green Party of England and Wales, said there were "lots of questions about the prime minister's conduct" and that he would support an inquiry, but argued the issue was a "huge distraction" from the main issues facing the public.
Dame Emily Thornberry said her committee was already investigating the appointment and that she did not want the Privileges Committee to be "duplicating the work we're doing".
"It may be that at some stage in the future, some of the questions haven't been answered, and it is decided that they are of sufficient importance that the Privileges Committee should be involved," she told BBC Radio 4's Today programme.
"But I don't really see why we're doing it at the moment, apart from, potentially people trying to score points in advance of the local elections."
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