Rishi Sunak provoked condemnation from a senior cabinet colleague and fury from Conservative grassroots after he was forced to apologise for skipping a key part of the D-day commemorations, the biggest misstep yet of an already faltering election campaign.

Penny Mordaunt, the leader of the House of Commons, criticised the prime minister’s absence from the international events as “completely wrong” during the BBC general election debate on Friday night, while Tory officials said internal party WhatsApp groups were full of horrified responses, with some local chairs saying the decision to go back to the UK to film a TV interview could cost the party seats on 4 July.

One senior grassroots member described the move as “extraordinary” and “gobsmacking”, with another saying: “No one is even trying to defend this. The belief was already very low in the election operation, and this does not help.”

Sunak sought to limit the damage with an early-morning tweeted apology, followed by a TV clip in which he said D-day commemorations should not be politicised.

In Sunak’s absence it was left to David Cameron, the foreign secretary, to take his place at Omaha beach in Normandy on Thursday afternoon, alongside Joe Biden, Emmanuel Macron and Olaf Scholz, the leaders of the US, France and Germany.

Ken Hay, who spoke at the commemorative event held at the British Normandy Memorial in Ver-sur-Mer, later said Sunak had let the country down. Photograph: Jane Barlow/PA

It is not clear why Sunak did not change his plans given the event for which he was returning, an interview with ITV News’s UK editor, Paul Brand, was not time-sensitive and will not be broadcast in full before next week.

“I stuck to the itinerary that had been set for me as prime minister weeks ago, before the election, fully participated,” Sunak said, insisting he “cared deeply” about commemorating what is likely to be the last major anniversary to include D-day veterans.

“As I said, on reflection it was a mistake not to stay longer and I’ve apologised for that, but I also don’t think it’s right to be political in the midst of D-day commemorations. The focus should rightly be on the veterans and their service and sacrifice for our country.”

However, the decision to leave early, when other world leaders and Keir Starmer stayed in Normandy for the entire series of commemoration events, has prompted despair at the tactical acumen of Sunak’s advisers, with Tory critics calling it a gift to both Labour and Nigel Farage’s Reform UK.

Rishi Sunak being interviewed by Sky News about leaving the D-day event early. Photograph: Sky News

Members were “disgusted”, the senior grassroots Tory said. “It’s firmed the idea that many Conservative voters are either now determined not to vote at all, or are going to vote for Reform.”

One local party chair, asked to describe the reaction, said: “Like a cup of cold sick would be generous. Association chairs are saying it could have electoral consequences in some constituencies. A lot are blaming the team around the PM.”

The veterans minister, Johnny Mercer, offered some defence, saying that while it was “a significant mistake for which he’s apologised”, he would not “join the howls of the fake veterans supporters who say he doesn’t treat veterans correctly, because it’s not correct”.

More awkwardly for the prime minister, one D-day veteran was damning. Ken Hay, 98, who was captured as a prisoner of war weeks into the operation, told Sky News: “He lets the country down.” Hay said Sunak had seemingly decided to “bail out, let them get on with it because ‘I want to stand in the election, I want my seat back.’”

There was more predictable condemnation from opposition parties, with Starmer saying Sunak’s decision jarred with his pledge to introduce mandatory national service, a plan intended to imbue 18-year-olds with a sense of civic duty.

Asked if there was a mismatch between the policy and Sunak’s actions, the Labour leader said: “I think there is, and he’s going to have to answer for the choices that he made. He’s the prime minister of the United Kingdom. This was a day of reflection, of being humbled really by the efforts that other people have made on our behalf.”

He added: “For me there was nowhere else I was going to be … There was only one choice, which was to be there, to pay my respects, to say thank you and to have to speak to those veterans.”

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The Liberal Democrat leader, Ed Davey, said: “One of the greatest privileges of the office of prime minister is to be there to honour those who served, yet Rishi Sunak abandoned them on the beaches of Normandy. He has brought shame to that office and let down our country.”

Farage tweeted that Sunak “could not even be bothered to attend” the Omaha Beach event.

The anger has dissolved any short-lived Tory buoyancy after a combative Sunak used Tuesday’s first head-to-head TV debate with Starmer to pummel the Labour leader over a much-disputed claim that the opposition’s spending plans would cost £2,000 a household.

This mood had already been dented by polls showing Farage’s decision earlier in the week to take over Reform’s campaign had given them a boost, with the party polling just behind the Tories.

Rishi Sunak and the French president, Emmanuel Macron, at a D-day celebration earlier in the day. Photograph: Chris Jackson/Reuters

With extrapolated polls suggesting a worse-case scenario for the Conservatives of winning only a few more MPs than the Liberal Democrats – in one projection, fewer – there has been increasing consternation at Sunak’s decision to call an election months earlier than was needed.

One regular complaint has been how unprepared the Tories appeared, despite choosing the election date, with a last-minute scramble to select dozens of candidates.

Even on Friday, with a 4pm deadline for nominations closing, the party had last-minute problems, with candidates replaced in Hemel Hempstead and Spen Valley after complaints about their conduct.

After a scramble to meet the deadline, the Tories managed to fill vacancies for all but one of the 631 seats they wanted to contest.

The outgoing Tory MP Jo Gideon showed the level of discontent within the party as she issued a long statement on Friday hitting out at bullying that she said had driven her out of politics.

She also criticised the party’s decision to pick “political careerist candidates” for the most winnable seats. “If the Conservative party finds itself in opposition, I hope they will use the opportunity to take a long, hard look at how they have allowed discrimination, bullying, and unethical practices to fester,” she said.