“After the conclusion of the British event in Normandy, I returned back to the UK,” Sunak said in a post on the social media site X.
“On reflection, it was a mistake not to stay in France longer – and I apologise.”
Sunak, languishing in the polls and widely tipped to lose the general election on July 4, sent his foreign minister David Cameron to the event instead, where he was pictured alongside other world leaders.
Sunak’s main opponent in the election, Labour leader Keir Starmer, did stay on and was photographed meeting Zelensky.
In a post on X, Starmer said he told the Ukrainian president that “there will be no change in Britain’s support for Ukraine” if he becomes the next UK prime minister as expected.
Starmer told reporters that Sunak would “have to answer for his own actions”, adding: “For me there was nowhere else I was going to be.”
Ed Davey, the leader of the smaller Liberal Democrats party, said Sunak had “brought shame” to his office and “let down our country”.
The prime minister’s own veterans affairs minister, former soldier Johnny Mercer, said he understood “the outrage” at Sunak’s actions and said his boss had committed a “significant mistake”.
Tory leader Sunak said in his post that the anniversary “should be about those who made the ultimate sacrifice for our country”.
“The last thing I want is for the commemorations to be overshadowed by politics,” he wrote. “I care deeply about veterans and have been honoured to represent the UK at a number of events in Portsmouth and France over the past two days and to meet those who fought so bravely.”
Sunak, 44, returned home to take part in an ITV News interview that is not due to air until Wednesday.
In a clip released late on Thursday, he was forced to deny that he is willing to lie to stay in power. Labour accuses him of lying by repeating a claim, disputed by independent experts, that Starmer would increase taxes by £2,000 (US$2,600) per household over four years.
The D-Day ceremonies marked the 80th anniversary of the launch of Operation Overlord, when tens of thousands of Allied troops stormed the beaches of Normandy in northern France on June 6, 1944. The vast military operation paved the way for liberation of occupied France and the end of the war against Nazi Germany.
Commentators suggested that Sunak’s decision to miss the main ceremony heralding the soldiers’ bravery showed a lack of political nous.
“It’s a very important moment for the country. But it’s also a very important moment to show that you’re being prime ministerial,” Cameron’s former adviser Craig Oliver told BBC radio.
Sunak, an internal Tory appointee as prime minister in October 2022, called the election in a rain-sodden speech from Downing Street on May 22.
Since then he has visited the site in Belfast where the Titanic was built, drawing comparisons between his leadership and captaining a sinking ship.
Sunak was also ridiculed for asking Welsh people if they were looking forward to the European Championships football tournament, when Wales have not qualified.