Keir Starmer would have blocked Peter Mandelson from serving as the UK’s ambassador to Washington had he known he failed security vetting, David Lammy has said, as he attempted to shore up the prime minister amid damaging fallout from the row.

In his first public comments on the vetting affair, Lammy said it was “inexplicable” that Oliver Robbins, the former top civil servant who was forced out of the Foreign Office this week, had opted to leave Downing Street in the dark over the outcome.

Starmer has been under intense pressure since the Guardian revealed on Thursday that the Foreign Office had overruled a decision to deny Mandelson security vetting clearance. He will face MPs on Monday as he attempts to limit the damage to his premiership.

Lammy, the deputy prime minister, told the Guardian he had been “shocked and surprised” when he had first learned last week what had happened.

But he insisted neither he nor his advisers at the time had known about – or asked for information on – the vetting process or its conclusions. Starmer and Yvette Cooper, the current foreign secretary, have also said they were unaware until this week.

Lammy did, however, admit there had been “some time pressures” on the Foreign Office last January to confirm Mandelson in post as Donald Trump was re-entering the White House.

“There was a feeling that obviously Trump had won the election in November, he was moving into the White House, and it would be good if we had an ambassador. So there was some time pressures around that I recall at the time.”

In his interview with the Guardian, Lammy said: “I have absolutely no doubt at all, knowing the PM as I do, that had he known that Peter Mandelson had not passed the vetting, he would never, ever have appointed him ambassador.

“The prime minister was not particularly close to Peter Mandelson. He hadn’t worked with him in the past, as some of us had. He was weighing a decision, but I’m quite sure had he known that, he would not have become ambassador. Therefore this is inexplicable.”

Lammy was on a military flight back from the Middle East when he was summoned to the cockpit by the captain who told him that No 10 needed to speak to him urgently over the radio.

“That obviously felt dramatic and serious and unexpected,” he said. “That was the first time I had heard this.”

Asked to clarify whether he or his advisers had been told about Mandelson’s vetting process, or had asked questions about it, during their time at the Foreign Office, he said: “No. And let me just be absolutely clear, in the years in which I have been both in this government and the last government, I have never had any official talk to me about vetting, or the detail of vetting, so I would remember if this had ever been raised with me.”

Robbins received the vetting outcome after Starmer had already announced Mandelson’s appointment and, despite a separate due diligence process, flagged the reputational risk of sending him to Washington.

Allies say he was simply following the rules to keep vetting from ministers, while some insiders have speculated the top civil servant was doing what he believed was expected of him at that stage. Others have suggested that mitigations were already in place after the due diligence report.

Lammy, who is also justice secretary and appointed Robbins to the role in January 2025, said he had found him to be an “outstanding” civil servant. “I don’t know what happened in this circumstance. I’m surprised and shocked by it. He was only a few weeks into the job,” he added.

The foreign affairs committee, meanwhile, published correspondence from the foreign secretary, Yvette Cooper, to the committee’s chair Emily Thornberry, which said: “I [the foreign secretary] have required the FCDO [Foreign, Commonwealth and Development Office] to conduct a full review of all the information that the department has given to your committee on the security vetting process undertaken as part of Lord Mandelson’s appointment.”

The senior Labour minister suggested next month’s local elections were likely to be tough for the Labour party even before the Mandelson row – with polls predicting a brutal set of results across the country – and he regretted it was back in the news.

They said: “It feels to me like these are tough midterm elections. I’ve knocked on a lot of doors up and down the country, particularly in London … There is a mood that has continued of cost of living pressures and angst and concern about public services.

“Local authority elections are to some extent a referendum on concerns about public services. I suspect the electorate will be communicating how they feel to all political parties, but certainly to the governing party, that’s what you would expect.”

Asked whether they would be even tougher as a result of the Mandelson scandal, Lammy said: “Clearly the fallout of Peter Mandelson’s behaviour and the decision to appoint him has … rumbled on for months and months and months.

“I regret that this is back again in the news when actually most people are worried about petrol prices, how the closure of the strait of Hormuz and the war in Iran is going to affect (them).”