Keir Starmer was left in the dark about sensitive information relating to Peter Mandelson’s security vetting by two other top civil servants, including the head of the civil service, the Guardian can reveal.

The prime minister said on Friday that it was “unforgivable” and “staggering” that senior officials did not tell him that Mandelson failed a security vetting process weeks before he took up his role as ambassador to Washington.

Olly Robbins was forced out of his job as permanent secretary of the Foreign Office on Thursday after it was revealed his department granted Mandelson developed vetting clearance against the advice of the relevant agency.

Now the Guardian can reveal that two other top civil servants, including the cabinet secretary, Antonia Romeo, failed to immediately notify him when they discovered that UK Security Vetting (UKSV) had advised that Mandelson should be denied clearance.

The Cabinet Office maintains that there was no undue delay because the civil servants were engaged in a process of “expedited checks” aimed at informing the prime minister as quickly as possible.

Downing Street has said Starmer did not find out about the vetting failure, which occurred in January 2025, until Tuesday this week. However, the Guardian has established that both Romeo, the government’s most senior civil servant, and Catherine Little, the Cabinet Office’s permanent secretary, have been aware since March.

Their delay in informing the prime minister will fuel concern about whether his government is being run by mandarins rather than ministers.

Romeo, who was appointed by Starmer in February, was told about the failure by Little in March. Little is the top civil servant at the Cabinet Office, which UKSV is part of. Her department has also been overseeing the process of complying with a “humble address”, parliamentary motion that ordered the government to release “all papers” relevant to Mandelson’s appointment.

The motion made an exception for papers prejudicial to national security or international relations, which it said should be released to the intelligence and security committee (ISC).

The cabinet secretary, Antonia Romeo (left), with Keir Starmer at a cabinet meeting in February. Photograph: Kin Cheung/AFP/Getty Images

A government source insisted Little “did not sit on the information” but was involved in a complex process and was trying to establish the risks in sharing highly sensitive information, including with the prime minister. The source added that Little informed Romeo of her plan to establish those risks. Romeo, the government source said, was supportive of the plan.

That process appears to have taken weeks, with as many as a dozen officials and lawyers aware of Mandelson’s vetting failure. Starmer’s statement would suggest he was not formally notified by any of them until a few days ago.

At the centre of the controversy was an extraordinary summary document produced by UKSV on 28 January last year, weeks after Starmer had announced Mandelson would be his ambassador to Washington.

The document identified highly sensitive concerns UKSV had about Mandelson and recommended, in conclusion, that he should not be given security clearance. It was that recommendation that was overruled by the Foreign Office.

A Cabinet Office spokesperson said that, on receiving the UKSV document after the humble address, Little “immediately undertook a series of expedited checks in order to be in a sound position to share the document, or the fact of it”.

The spokesperson said this included receipt of legal advice about what could be shared in the context of the humble address and consideration of whether the information would prejudice criminal proceedings.

Little also sought information from the Foreign Office about “the process they had followed” when deciding to give Mandelson security clearance against the advice of UKSV, the spokesperson said. They added: “As soon as these checks were conducted, the prime minister was informed.”

According to a government source, Little had always been of the view that the outcome of the UKSV process should be made public, and the relevant document disclosed in unredacted form to the ISC. However, officials in her department have in recent weeks been divided over how to proceed and whether to release the document to the committee at all.

Peter Mandelson photographed near his home in London on Friday. Photograph: Chris J Ratcliffe/Reuters

Prior to the publication of the Guardian’s story on Thursday, there was said to have been “no consensus” among officials. Some flagged national security concerns and argued it would be “unprecedented” to disclose the UKSV file, even to the ISC, a committee comprising nine MPs and peers, including Jeremy Wright, a former attorney general, and Alan West, a retired Royal Navy admiral.

Its members are sworn to secrecy under the Official Secrets Act and are given access to highly classified material. According to one source familiar with debates swirling in Little’s department, there were fears among at least some officials that there might be an attempted “cover-up” and the document would never see the light of day.

Some officials noted that the UKSV document appeared to contradict statements made by the prime minister and his former chief of staff, Morgan McSweeney, that implied vetting failures could partly be blamed for Mandelson’s appointment.

Amid an impasse among officials, some in government are said to have argued that precedent should be set aside to disclose the UKSV documents to the committee, and tjat anything short of that would risk breaching the wishes of parliament.

The discussion about whether or not to release the documents to the parliamentary committee appears to have lasted for weeks. If Downing Street’s chronology is to be believed, the prime minister was completely oblivious that it was even happening.

By Wednesday this week, one compromise option being considered involved providing unredacted versions of the document only to two ISC members, such as the chair and one other member. Another was only showing the documents to those members of the committee who were also members of the privy council, a historical body that advises the monarch.

One source said Little was now expected to be asked to appear before the ISC in a closed hearing to answer questions. Lord Beamish, who chairs the ISC, has said the committee and parliament would take a “very dim view” if documents were withheld from its members.

A Cabinet Office spokesperson said Little and officials working on the humble address “have always worked on the basis of being transparent about the UK Security Vetting recommendation”.

Neither the Cabinet Office nor No 10 have disputed, however, that there has been an internal debate over whether the materials could be withheld. That raises questions about the accuracy of public remarks on Friday by the chief secretary to the prime minister, Darren Jones.

A close ally of Starmer, Jones was asked on the BBC’s Today programme to comment on the Guardian’s report that “officials have toyed with the idea at least of not revealing all of this to parliament”.

He replied: “That’s not true. All of these documents are going through what’s called the humble address process, which my department is responsible for.”

Asked if he had misled the public, a source close to Jones insisted that his answer was “clearly focused on the official government response to the humble address, which he makes clear later in his answer”.

On Friday evening, Downing Street published a summary of the meeting held on Tuesday when Starmer was told Mandelson had been appointed despite failing to clear security vetting.

The email, from his principal private secretary Dan York-Smith, said: “The PM was not aware of any of this before the meeting, including that it was even possible to grant clearance against the advice of UKSV.”

It shows he instructed officials to establish facts so he could update MPs as soon as possible.