Keir Starmer did not gamble with national security by appointing Peter Mandelson as the UK’s ambassador to Washington, a cabinet minister has said, as the government tried to limit fallout from the scandal.

With the prime minister preparing for a high-stakes Commons showdown on Monday, Liz Kendall said Starmer should not lose his job over last week’s revelations because he had “made the right calls” on the big issues facing the country.

The technology secretary warned Labour MPs who might be considering a leadership bid that, with a cost of living crisis and global uncertainty, they would lose public support if they prioritised their own futures rather than the country’s.

Starmer has been grappling with the Guardian’s revelation that Mandelson was appointed despite failing security vetting, which led to the sacking of Oliver Robbins, the Foreign Office’s top civil servant.

Ministers have been trying to shore up Starmer’s position, with David Lammy, the justice secretary, telling the Guardian the prime minister would have blocked Mandelson from the role had he been aware of the vetting outcome.

David Lammy has been among ministers trying to shore up Keir Starmer’s position. Photograph: Jordi Matas/The Guardian

In an interview on Sky News’s Sunday Morning with Trevor Phillips programme, Kendall was asked whether – given Mandelson was always regarded as a potentially risky appointment – Starmer had gambled with national security by approving him.

She replied: “No, we did not. I completely refute that the prime minister would do anything to put the UK’s security at risk. The precise opposite. And of course people knew that Peter Mandelson had been sacked from the cabinet twice. I mean, that’s a statement of the fact.”

Kendall echoed Lammy by saying that Starmer would not have gone ahead with the posting had he known Mandelson did not receive clearance from the government vetting service. “He would not have made that appointment. I think it’s just really important to be clear about it,” she said.

She was challenged over whether Starmer’s poor judgment in appointing Mandelson, whose links with the convicted child sex offender Jeffrey Epstein were known at the time, was the root of the issue. Kendall said: “I don’t agree with that. I think the failure of judgment here was the failure to tell the prime minister that Peter Mandelson, who actually is responsible for all of this … the person that I am angry at is Peter Mandelson.”

Keir Starmer has shown “catastrophic misjudgment’, according to Ed Davey, the leader of the Liberal Democrats. Photograph: Jeanne Accorsini/SIPA/Shutterstock

Asked why Starmer was not losing his job over the scandal, as a growing list of civil servants have done, she said it was because the prime minister, “on the big calls facing this country, has made the right calls”, including on global issues.

On Friday, the Cabinet Office released a template page from the summary document produced by the UK Security Vetting (UKSV) after Mandelson went through the process. The document would be used by a vetting officer to summarise their findings.

It lists three rankings for possible overall concern: low, medium and high. In the next box, there is a space for a vetting officer to list the outcome of the assessment with their overall decision or recommendation. Again, there are three options: clearance approved, clearance approved “with risk management”, or clearance denied.

According to multiple sources, the vetting process in Peter Mandelson’s case concluded there was a ‘high’ overall concern. Photograph: Chris J Ratcliffe/Reuters

According to multiple sources, the UKSV process in Mandelson’s case concluded there was a “high” overall concern and concluded “clearance denied”. It was this recommendation that was overruled by the Foreign Office, which employed a rarely used authority to grant him clearance anyway.

It has been speculated that Mandelson had agreed to some mitigations that were proposed by the government, relating to his former commercial clients. However, these were likely to have been as a result of a separate Cabinet Office due diligence process that said there was a reputational risk in pressing ahead with the appointment.

Alex Burghart, a Tory frontbencher, accused Starmer of lying and suggested the Conservatives could consider laying a no confidence motion in the prime minister. “We have a number of tools in our parliamentary arsenal, and we will choose our moments appropriately,” he added.

Ed Davey, the leader of the Liberal Democrats, said Starmer had shown “catastrophic misjudgment” while Reform’s Treasury spokesperson Robert Jenrick said: “He should be gone.”