A former head of MI6 has said it would have been “totally impossible” for the Foreign Office to put in place mitigations to manage Peter Mandelson’s associations with senior figures in China, Russia and Israel when he was the UK’s ambassador to the US.

On Wednesday, the Guardian revealed some of the concerns that contributed to security officials recommending that Mandelson be denied developed vetting clearance in early 2025.

Olly Robbins, the former permanent secretary who granted Mandelson clearance anyway, has said his decision was based on “management actions” being put in place to mitigate the risks identified by officials.

However Richard Dearlove, who ran Britain’s Secret Intelligence service, MI6, between 1999 and 2004, said he did not understand how such mitigations would work.

“The whole thing is completely extraordinary,” he said. “What mitigations could you put in place? The only mitigations I can think of is that certain papers that are circulated in the Washington embassy cannot be viewed by the ambassador. That would be totally impossible.”

Dearlove added: “When I was head of MI6, if I’d been warned not to share papers with a minister or ambassador, I’d have asked to discuss the situation with the foreign secretary or the prime minister.”

Multiple sources have told the Guardian that Mandelson’s links to China’s minister of finance, Lan Fo’an, the sanctions-hit Russian oligarch Oleg Deripaska and former Israeli military intelligence general Tamir Hayman were all flagged by the government’s vetting agency.

They added that United Kingdom Security Vetting (UKSV) also noted Mandelson had a very close relationship with a fourth individual, who is British, that could be compromising. Another concern identified by the vetting agency, the sources said, was a £1m loan Mandelson received to invest in an Israeli startup.

UKSV recommended denying Mandelson clearance, but within hours the Foreign Office granted it to him anyway.

Emily Thornberry, the chair of the foreign affairs select committee, said the revelations made her “very angry” and raised questions about comments Robbins made in evidence to her committee, when he said that UKSV had regarded Mandelson’s case as “borderline”.

“It becomes quite clear why UKSV saw [Mandelson] as a subject of concern who shouldn’t be granted clearance. It makes Olly Robbins’ assertion that he understood the recommendation to be ‘borderline’ pretty incredible.” Robbins has not responded to requests for comment about the evidence for his “borderline” claim.

Next month the government is due to publish a tranche of documents after parliament passed a motion known as a humble address, ordering the release of all papers relating to Mandelson’s appointment. However, there are growing concerns that not all relevant documents will be released in full.

On 15 May, the intelligence and security committee, which has a role in reviewing the most sensitive of those documents, publicly accused the government of withholding some Mandelson vetting documents and implementing redactions “far too broadly”. During a debate in parliament last week, MPs from across the political divide criticised the Cabinet Office’s handling of the process.

Two MPs openly accused the government of a “cover-up” and one threatened to table a motion holding ministers in contempt.

Responding to the Guardian’s revelations, Ed Davey, the leader of the Liberal Democrats, said: “The fact that the government is still trying to hide the truth when it comes to Mandelson is an utter disgrace. I can only imagine how angry Keir Starmer would be about it if he wasn’t running the government.

“The responsibility for the hiring of Peter Mandelson lies at the door of the prime minister. No 10 knew full well about Mandelson’s business relationships with China and Russia, and indeed with [the child sex offender] Jeffrey Epstein too.

“Glaring warning signs were wilfully ignored, driven by a desperate desire to pander to the bully in the White House rather than protect British interests.”

The shadow foreign secretary, Priti Patel, said: “These shocking revelations underline just how reckless Keir Starmer’s decision to appoint Peter Mandelson was. A man with these links to Russia and China should never have been handed one of our most sensitive diplomatic posts.”

During the parliamentary debate, Darren Jones, the Cabinet Office minister overseeing the humble address process, defended the government’s right to make its own redactions and withhold some of the most sensitive vetting files from parliament. He also denied there had been a cover-up.

A spokesperson for Hayman said he “has no personal connection or familiarity whatsoever” with Mandelson, and his contact with him was limited to work the British politician did for his thinktank.

A Cabinet Office spokesperson said it was “committed to complying with the humble address in full”.

The Foreign Office and representatives for Mandelson, Robbins, Lan and Deripaska have all been contacted for comment.