Calls were growing on Thursday for Keir Starmer to face a Parliamentary inquiry for allegedly misleading the Commons about the Peter Mandelson affair.
The Prime Minister had already been accused of wrongly assuring MPs that full due process had been followed in the appointment of the Labour grandee as US Ambassador.
But now he has been hit by further damaging allegations that he misquoted the mandarin he sacked over the scandal.
Sir Keir told Prime Minister's Questions that Sir Olly Robbins had said his decision on whether to grant Lord Mandelson security clearance was 'rigorously independent of any pressure'.
But in fact Sir Olly had told the Foreign Affairs Committee that his decision was made 'rigorously independent of that pressure'. Earlier he had revealed his office had been under 'constant pressure' from No 10 to approve the appointment.
On Thursday, a suspended Labour MP joined the demands from opposition parties for the Prime Minister to be investigated by the Privileges Committee - the same body that probed Boris Johnson over Partygate - for allegedly misleading Parliament.
Karl Turner revealed he had written to Commons Speaker Sir Lindsay Hoyle requesting he consider a referral.
Under Parliamentary procedure, the Speaker must agree to a vote and MPs must back the calls for an investigation before it can take place.
Sir Keir Starmer told Prime Minister's Questions that Sir Olly Robbins had said his decision on whether to grant Lord Mandelson security clearance was 'rigorously independent of any pressure'
Karl Turner (pictured) insisted he was not accusing Sir Keir of deliberately misleading Parliament but said there are evident inconsistencies between statements made at Prime Minister's Questions and the evidence given by Sir Olly Robbins to the Foreign Affairs Committee
Despite Labour's large majority, some suspect that many of the party's MPs could refuse to back Sir Keir because of their anger over the Peter Mandelson scandal, which has now dogged the PM for more than seven months.
A Privileges Committee inquiry would be independent of Government, which is in charge of the separate process of publishing files on the Mandelson scandal. It stands more of a chance of going ahead than an investigation by the ministerial sleaze watchdog into whether Sir Keir has broken the Ministerial Code, of which the PM is the ultimate judge.
The Privileges Committee is chaired by a Conservative MP although four of its seven members are Labour.
If it set up an inquiry, it would be able to summon witnesses from the Government to hearings and demand documents including emails and WhatsApp messages. It would also question Sir Keir under oath, as it did to his Tory predecessor Mr Johnson, and would have the power to suspend him from Parliament if he was found to have misled the House and committed a contempt.
Mr Turner insisted he was not accusing Sir Keir of deliberately misleading Parliament but went on: 'There are evident inconsistencies between statements made at Prime Minister's Questions and the evidence given by Sir Olly Robbins to the Foreign Affairs Committee. These raise important questions that go to the heart of transparency and accountability in Parliament. It is right that they are examined properly.'
The Conservatives and Liberal Democrats are already in favour of the probe. Shadow Chancellor Sir Mel Stride told GB News: 'Our belief is that the Prime Minister has misled Parliament, because what he said in Parliament is that pressure was not applied, and he's been contradicted by Olly Robbins.'
Sir Philip Barton will be asked about the claim he was ordered by No 10 to 'just f***ing approve' the appointment - and if he quit his job early because of the pressure.