Sir Keir Starmer would have blocked the appointment of Lord Mandelson as US ambassador had he known he failed security vetting, ministers have said.
The prime minister has come under pressure after it emerged Mandelson was appointed despite security concerns, with No 10 saying red flags in the vetting process were not disclosed by the Foreign Office.
Technology Secretary Liz Kendall said on the BBC's Sunday with Laura Kuenssberg that Sir Keir was told Mandelson had been granted developed vetting status, adding: "If he had known that UK security vetting hadn't cleared him, he would not have made that appointment."
Sir Keir is set to face MPs on Monday to face questions about the appointment.
Opposition parties have called on the prime minister to resign, accusing him of misleading Parliament over his previous statements that due process had been followed in relation to the appointment.
Appearing on the BBC's Sunday with Laura Kuenssberg programme, Kendall was asked why Sir Keir had not updated the Commons since being made aware of Mandelson's vetting failure on Tuesday.
She said: "I think one thing we've learned from this whole torrid episode is the need to get the facts absolutely clear and right."
Kendall's defence of Sir Keir echoed remarks by Deputy Prime Minister David Lammy, who told the Guardian he had "absolutely no doubt at all" that the prime minister "would never, ever have appointed" Mandelson if he had known he failed vetting.
Lammy, who was foreign secretary at the time of Mandelson's appointment, said that neither he nor his advisers had been told about the vetting process.
The deputy prime minister said he was "surprised and shocked" by the departure of Sir Olly Robbins, the Foreign Office's most senior civil servant who was ousted this week over the vetting row.
Lammy pointed out Sir Olly had only been in the job for a few weeks when the vetting report was returned, and said there were "time pressures" on the Foreign Office to have Mandelson in place shortly after Donald Trump's return to the White House.
His successor in the Foreign Office, Yvette Cooper, previously confirmed Mandelson's vetting was made a "priority clearance", but insisted full checks were still carried out despite the process being fast-tracked.
Sir Keir has said it was "staggering" he had not been told sooner that Mandelson failed security vetting, which only began after the former minister was picked to be the UK's representative in Washington.
Speaking on Sunday, former senior civil servant Helen MacNamara said she did not think Sir Olly should have been sacked, adding: "One of the many frustrating things about this is that there's still no information."
She said the government had tried to find "new processes to blame, new people to blame" for the ill-fated Mandelson appointment.
Speculating about why Foreign Office officials awarded the clearance, she said it was possible there was a view the risks associated with Mandelson were "priced in", and it was for them to do "what the prime minister wanted", before putting in place "any mitigations in place to make sure there weren't any security concerns".
Also appearing on Sunday with Laura Kuenssberg, Conservative shadow Cabinet Office minister Alex Burqhart said: "I think there's one person who's responsible for everything that's happened, and that's the prime minister."
Robert Jenrick, Reform's Treasury spokesperson, said: "I don't know whether Keir Starmer is a liar or just grossly incompetent - what I do know... is that he's totally unfit to lead this country."
Liberal Democrat leader Sir Ed Davey, appearing on Sky's Sunday Morning with Trevor Phillips, said Sir Keir has shown "catastrophic misjudgment" on "many levels".
Dame Emily Thornberry, chair of the Foreign Affairs Committee, has said new revelations have "called into question" evidence Sir Olly gave to MPs in November, during which he did not disclose that the government's security vetting agency advised the Foreign Office to deny Mandelson a high-level security clearance.
Sir Olly is expected to be questioned by the Foreign Affairs Committee again on Tuesday.
His allies have said this week that he was bound by the confidential nature of the intrusive vetting process, hence why it was not disclosed to Downing Street at the time.
BBC News understands Sir Olly has not formally accepted the committee's invitation to give evidence, but friends of his said he was preparing to appear on Tuesday.
On Saturday, Cooper told the committee she had asked for a review of the information given to MPs by officials to ensure it was "fully accurate".
BBC News understands that those close to Mandelson believe the sacking of Sir Olly as permanent secretary is "egregious".
Cooper has also confirmed that Nick Dyer, who has held a number of other senior roles in government, had been asked to run the Foreign Office civil service on an interim basis.
