"After the conclusion of the British event in Normandy, I returned back to the UK," Sunak - who is struggling to win support from voters before the July 4 election - said in a post on X.

"On reflection, it was a mistake not to stay in France longer – and I apologise."

World leaders including U.S. President Joe Biden and Britain's King Charles gathered in Normandy, northern France, to mark the 80th anniversary of the allied landings, a turning point in World War Two.

Sunak spoke at a British-led event but delegated other duties to ministers including Foreign Secretary David Cameron, who was pictured with Biden, French President Emmanuel Macron and German Chancellor Olaf Scholz at a later memorial ceremony.

News reports said Sunak left France early in order to conduct a television interview with a British broadcaster.

Sunak's Conservative Party is lagging about 20 points behind the opposition Labour Party in opinion polls ahead of the national election.

Labour Party leader Keir Starmer also attended events in Normandy on Thursday where he met Ukrainian President Volodymyr Zelenskiy.

"Yesterday's D-Day commemorations were about remembering the bravery of all those who serve our country," senior Labour spokesman Jonathan Ashworth said.

"In choosing to prioritise his own vanity TV appearances over our veterans, Rishi Sunak has shown what is most important to him."

Liberal Democrat Leader Ed Davey accused the prime minister of a "total dereliction of duty".

Sunak's campaign got off to a shaky start last month when he announced the election date under a downpour of rain competing to be heard against Labour supporters blaring the song associated with the party's crushing 1997 election victory.

This week he suffered another setback when Brexit campaigner Nigel Farage took over leadership of the right-wing Reform UK party and said he would stand in the election.

An opinion poll published on Wednesday showed support for Reform was just two percentage points short of the Conservatives.