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The evidence of two ex-officials on Tuesday morning will be a key moment in the growing crisis around Peter Mandelson’s vetting for US ambassador which threatens to bring down Keir Starmer’s premiership.

The prime minister will later face a crucial vote on a parliamentary inquiry into whether he misled MPs when claiming “full due process” had been followed.

Morgan McSweeney, Keir Starmer’s former chief of staff, and Philip Barton, who was the top official in the Foreign Office (FCDO), have been called by the foreign affairs committee after the Guardian’s revelation that Mandelson was given security clearance despite vetting officials recommending it be denied.

Their testimonies will follow an explosive parliamentary session last week. Starmer told the Commons last Monday that he and other ministers had not been told Mandelson had failed the “developed vetting” process carried out by United Kingdom Security Vetting (UKSV). He pointed the finger at Olly Robbins, who succeeded Barton as the top civil servant in the FCDO in January 2025.

After the Guardian broke the news that the Foreign Office had overruled UKSV, the prime minister sacked Robbins. Robbins gave his side of the story last Tuesday.

So what questions might Barton and McSweeney face?

Was due process followed?

At the heart of the charges levied by opposition MPs against Starmer is his claim that “full due process” was followed in Mandelson’s appointment, first made last September and which the prime minister last week said he stood by.

As key officials in the Foreign Office and Downing Street around the time Starmer made the decision to appoint Mandelson to Britain’s top diplomatic posting, Barton and McSweeney are well placed to comment on questions of due process.

McSweeney has already said he takes “full responsibility” for advising Starmer to appoint Mandelson – but that is a question of judgment, not process.

Who was arguing that Mandelson did not require vetting?

According to Robbins, Barton had to be “very firm in person” with the Cabinet Office to insist that Mandelson go through the developed vetting process. Nearly all of the staff, including junior civil servants, in the FCDO are required to have this level of security clearance. So it would have been remarkable for Mandelson not to have done so.

Robbins said, however, there was a “position taken from the Cabinet Office” that, as Mandelson was a peer and a privy counsellor – a mostly honorary role given to senior ministers which gives access to certain briefings – there was no need. But the Cabinet Office has challenged this and suggested it was the FCDO that proposed there was no need to vet Mandelson.

Barton could explain his role in that debate. So too might McSweeney, who had a hand in the due diligence process carried out by the Cabinet Office before Mandelson was announced by Starmer.

When McSweeney resigned in February 2026, he said: “While I did not oversee the due diligence and vetting process, I believe that process must now be fundamentally overhauled.” What was it about those processes that McSweeney felt should be overhauled?

What pressure did the FCDO come under to get Mandelson in post as soon as possible?

At prime minister’s questions, Starmer sought to use Robbins’s testimony to dismiss claims Downing Street had put pressure on the FCDO to get Mandelson to Washington as soon as possible. Starmer said: “No pressure existed whatsoever in relation to this case.”

But Robbins said: “Throughout January, honestly, my office and the foreign secretary’s office were under constant pressure.” He characterised the questions from Downing Street as being about “when” Mandelson would be in post, not “whether” he might.

Robbins added: “Philip’s [Barton] handover to me has contributed to my strong sense that there was an atmosphere of pressure and a certain dismissiveness about this DV process”. Barton will probably be asked to give further details.

McSweeney may be asked to explain whether he had any role in putting pressure on officials in Downing Street to chase the FCDO.

Why was Mandelson given access to the FCDO before his vetting?

Before Mandelson had completed vetting, he was given access to “higher-classification briefings” and a pass to the FCDO’s building. Photographs of Mandelson from mid-January seem to suggest the pass was awarded towards the start of the month, while Barton was still in post.

One former official, familiar with security processes across the British state, has told the Guardian that such a pass being given on an interim basis would be highly unusual, as it gives access to classified areas across government, not just in the FCDO.

Barton may face questions on any role he played in facilitating access for Mandelson before his clearance. Was it due to any pressure or particular expectations from Downing Street?

But can either man give further details on why Mandelson was given vetting?

Probably not. The FCDO’s head of security, Ian Collard, was also called to give evidence to MPs. But he will not appear in person. He has been sent questions by the foreign affairs committee, which describes him as “the only person who was involved in some of the process which granted Lord Mandelson DV clearance”, asking for further details of the crucial 29 January 2025 meeting with Robbins.

Perhaps, given both have departed government, Barton and McSweeney could explain why this might be the case. What advice was given to ministers that led to the foreign secretary telling MPs that Collard could not appear in front of them? He has, after all, previously given evidence to a different committee while in the same role.