Keir Starmer has said it was “unforgivable” that he was not told that Peter Mandelson had failed his security vetting before taking up his role as ambassador to Washington.

The prime minister said he was “furious” about what had happened, as he insisted he had not known that security officials had recommended that Mandelson be denied clearance.

Answering questions later on, Starmer’s spokesperson squarely blamed the Foreign Office, saying Downing Street had “repeatedly” sought the facts of the case but was not told.

Asked if this amounted to a “cover-up”, the spokesperson did not reject this, saying: “Well, the prime minister was not informed and he’s made clear that it is staggering that he was not informed.”

Speaking on Friday morning for the first time since the Guardian broke the story, Starmer said: “That I wasn’t told that Peter Mandelson had failed security vetting when he was appointed is staggering.

“That I wasn’t told that he had failed security vetting when I was telling parliament that due process had been followed is unforgivable. Not only was I not told, no minister was told, and I’m absolutely furious about that.”

The prime minister is under pressure to explain who knew what about Mandelson’s vetting, after the Guardian revealed on Thursday that the former peer had failed the checks.

Mandelson was appointed to the post in late 2024, despite concerns among senior members of the government about his relationship with Jeffrey Epstein even after the financier had been convicted of child trafficking.

He was sacked by Starmer last year after emails were published by the US Department of Justice showing how close he had been to Epstein, to the extent that he had apparently shared sensitive government information with him while working for Gordon Brown. He denies any wrongdoing.

Sources have told the Guardian that security officials recommended that Mandelson not be given security clearance for the role, but that they were overruled.

It remains unclear who gave the order for the clearance to be granted. Starmer sacked Olly Robbins, the most senior civil servant in the Foreign Office, on Thursday night.

The prime minister will appear before MPs on Monday to give a statement about what happened and what he knew. He said on Friday: “What I intend to do is to go to parliament on Monday to set out all the relevant facts in true transparency, so parliament has the full picture.”

Questioned at length at a media briefing about what had happened, Starmer’s spokesperson said the prime minister had ordered an urgent inquiry, the terms of reference for which would be announced soon.

“The prime minister was notified about this on Tuesday evening,” he said. “He asked the cabinet secretary to establish the facts in order that he could update parliament, as soon as is possible.” That work was “ongoing”, the spokesperson said.

He added: “Nobody in No 10 was told. Nobody – No 10 officials or otherwise – had this information.”

Asked about whether No 10 had sought to interrogate the Foreign Office about Mandelson’s vetting, the spokesperson said: “What I can say is that those conversations have happened multiple times throughout this process. No 10 has repeatedly asked for the facts of the case and at no point has the fact that UK Security Vetting recommended against the vetting [been] provided to No 10.”

Robbins could give his side of the story early next week, with the Commons foreign affairs select committee inviting him to give evidence on Tuesday.

Speaking to the BBC, Ciaran Martin, a former senior civil servant with past involvement in vetting work, who is a close friend of Robbins, said Robbins appeared to have been made a scapegoat.

Martin said vetting had been wrongly presented as a simple pass or fail, when it was instead a “risk assessment”, and that it was entirely standard for officials to decide whether the balance of risk was acceptable.

“The one thing you never do is tell ministers of any kind, because otherwise the vetting system would collapse,” he told BBC Radio 4’s World at One.

“If ministers make senior appointments and you go and say, ‘Look, we’ve delved into [it] and this is their money, this is their private life’ and so forth, then nobody would undertake vetting.”

Starmer has been accused of misleading parliament, given that he repeatedly assured MPs that “full due process” had been followed before Mandelson was given the job.

Asked if Starmer had done this inadvertently, his spokesperson said: “The prime minister feels that he should update parliament on Monday on the basis that parliament should have known about this and should now know about this.”

Kemi Badenoch, the Conservative leader, has led calls for the prime minister to resign over the controversy. She said on Friday: “It is completely preposterous for us to believe that civil servants would have cleared a political appointee who had failed security vetting.”