Rishi Sunak has apologised for missing a key part of the D-day commemorations in northern France to film a TV interview, as he faces a wave of condemnation over what may be his biggest misstep yet in a faltering election campaign. The prime minister was heavily criticised for leaving the 80th anniversary events for an ITV interview that is not scheduled for broadcast until next week, with opposition parties calling it crass and a dereliction of duty. He was also forced to deny he had planned to skip the D-day commemorations altogether, as he called for the events not to be politicised. In a slightly convoluted-sounding explanation, Sunak said he did not prioritise the election over what is likely to be the last major anniversary to include D-day veterans – saying his itinerary had been set before the election was called. But it remained unclear why he returned to the UK early for the interview, leaving David Cameron, the foreign secretary, to take his place for the late afternoon ceremony at Omaha beach on Thursday, alongside Joe Biden, Emmanuel Macron and Olaf Scholz, the leaders of the US, France and Germany. With many Conservatives privately dismayed at the decision, and criticism mounting overnight, Sunak tweeted an early-morning apology on Friday. “After the conclusion of the British event in Normandy, I returned back to the UK. On reflection, it was a mistake not to stay in France longer – and I apologise,” he wrote. He sidestepped accusations that his actions showed a disdain for the military, instead claiming his record shows he has long supported the armed forces with “increasing levels of investment”. It was subsequently reported that the French government had claimed its officials were told last week that Sunak would not attend the commemorations at all because of the general election campaign. In a broadcast interview later on Friday, the prime minister denied the election had been a factor: “I stuck to the itinerary that had been set for me as prime minister weeks ago, before the election, fully participated. “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.” The Labour leader, Keir Starmer, said Sunak’s decision to leave Normandy early jarred with his pledge to introduce mandatory national service – intended to imbue 18-year-olds with a sense of civic duty. A No 10 spokesperson said: “The prime minister was always scheduled to attend D-day commemorations, including the UK national commemoration event in Normandy, and it is incorrect to suggest otherwise.” Conservative activists reacted with fury to Cameron’s presence at the afternoon event, with one saying it had left them questioning whether to “bother to continue campaigning”. The veterans minister, Johnny Mercer, told the Sun: “I get the outrage. It’s a mistake. It’s a significant mistake for which he’s apologised.” Sunak’s apology also came after the ITV presenter Paul Brand confirmed on News at Ten that the prime minister had returned from Normandy to speak to him. Brand said ITV was interviewing all of the party leaders and had been working to secure a date with Sunak for some time. “Today was the slot they offered us,” he said. “We don’t know why.” Opposition politicians criticised Sunak on Friday morning, saying he had “brought shame” on the office. When asked if there was a mismatch between Sunak’s national service policy and his behaviour on the 80th anniversary of D-day, Starmer told the Guardian: “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.” 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. “I am thinking right now of all those veterans and their families he left behind and the hurt they must be feeling. It is a total dereliction of duty and shows why this Conservative government just has to go.” The Reform UK leader, Nigel Farage, tweeted that Sunak “could not even be bothered to attend” the Omaha Beach event. One Tory activist said: “Lots of us are asking each other what’s the point, across associations. He allowed a former PM to get some pics next to Biden in his place. Why should we bother to continue campaigning, knocking on hundreds of doors when Sunak seems to be doing all he can to completely ruin our chance of losing in a way that’s just about tolerable.” The shadow defence secretary, John Healey, has sent a letter to his cabinet colleague Grant Shapps asking when the decision for Sunak to skip the commemorations was made. He also queried whether the French government was correct in saying they were told a week ago that the prime minister would not attend at all. Sunak was present at an event on Thursday morning at Ver-sur-Mer, in Normandy, which was also attended by Macron, King Charles and Queen Camilla.