Sir Keir Starmer was facing mounting criticism from Labour ministers as a second senior civil servant was grilled about Lord Peter Mandelson's vetting process.
Cat Little, the most senior Cabinet Office official, appeared before MPs on Thursday morning and insisted due process was followed when vetting the peer for his post as US ambassador.
It comes after sacked Sir Olly Robbins appeared before the Foreign Affairs Select Committee on Tuesday to give damning evidence about the role Downing Street played in the appointment of Mandelson.
Cabinet divisions have emerged over the Prime Minister’s handling of the scandal, including his decision to fire Foreign Office chief Sir Olly for failing to tell him about security concerns and suggestions the PM’s ex communications chief was being considered for a top diplomatic job, according to multiple reports.
Sir Olly Robbins revealed that Lord Matthew Doyle, who earlier this year lost the Labour whip for campaigning for a convicted paedophile, had been considered for an ambassadorship, a disclosure Foreign Secretary Yvette Cooper said made her “extremely concerned”.
Labour backbenchers have openly voiced doubts about Sir Keir's future since it emerged last week that the Foreign Office decided to appoint Mandelson despite the fact he failed the vetting process.
Ms Little told the Prime Minister last week that Mandelson had been granted high-level security clearance after vetting officials recommended against it.
She had known since March about sensitive information linked to the vetting.
On Thursday she told the Foreign Affairs Committee that Sir Olly would not share some documents related to Mandelson's appointment as US ambassador.
Ms Little said she had begun to uncover documents related to Lord Mandelson as part of the humble address process which began in February and is aimed at providing detailed information to MPs about the process which led to the peer becoming the US ambassador.
The senior civil servant told the committee she had "multiple discussions" with then-Foreign Office chief Sir Olly as part of trying to gather information for the humble address, before adding: "In the middle of March, I have a meeting with Sir Olly and a senior member of his team, and this is after the point that I've been told that this summary document exists.
"I specifically ask to see this document and any decision-making audit trail around those judgments at the time. It was made clear to me that that information would not be forthcoming."
Asked by committee chairwoman Dame Emily Thornberry who was not forthcoming, Ms Little replied: "Sir Olly."
This was in the context of "a lot of back and forth discussion about the status of how we would treat vetting information", Ms Little said.
Downing Street previously said there is "absolutely no suggestion" that Ms Little's position as Cabinet Office permanent secretary is in question.
A junior minister on Thursday said claims of a Cabinet split over the scandal are "a load of guff".
Home Office minister Alex Norris also told LBC: "If I had a pound, certainly under the previous government, for the number of times I saw cabinet stories in the papers, my St George's pints would probably be more multiple than there will be in reality."
Responding to calls from Labour MP Jonathan Brash for the Prime Minister to step aside, Mr Norris insisted to Sky News that Sir Keir would lead Labour into the next election.
The minister added: "What I would say to Jonathan and to others is, look at what we delivered this month alone."
He listed "huge decreases" in NHS waiting lists and lifting the two-child limit on benefits as among recent action by the Government, adding: "This is a Government that's getting on with change, spearheaded by our Prime Minister, and we should be all pulling in that direction."
On Wednesday, Work and Pensions Secretary Pat McFadden repeatedly declined to say whether he believed the sacking of Sir Olly was fair.
As pressure mounts against the Prime Minister, Conservative leader Kemi Badenoch is said to be pressing for his referral to the Privileges Committee claiming he misled MPs, the Times reported.
The same committee found former Prime Minister Boris Johnson lied to the Commons over so-called partygate.
The Tory leader has suggested Sir Keir misled the Commons when he said "due process" had been followed in the appointment of Lord Mandelson as British ambassador to the US, according to the newspaper.
He has denied misleading MPs.
Sir Keir's former chief of staff Morgan McSweeney has also been summoned to appear before the Foreign Affairs Committee next week.
The row over Sir Olly's sacking dominated the Prime Minister's Questions clash with Kemi Badenoch.
He insisted Sir Olly's evidence about Lord Mandelson exonerated him over accusations that he had misled MPs.
The mandarin's testimony "puts to bed all the allegations levelled at me by those opposite in relation to dishonesty", the Prime Minister said.
