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”.
“ULTIMATE SACRIFICE”
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 be quizzed by ITV News in an 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 Jun 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.
Labour was finalising its manifesto on Friday, with Starmer confirming that recognition of Palestinian statehood as part of any Middle East peace process would be included.