UK Prime Minister Keir Starmer paid tribute to US President Joe Biden in Britain's parliament on Monday, saying America's leader will leave NATO "stronger than it's been for decades".

Biden, 81, announced on Sunday that he was dropping out of the US presidential race following intense pressure to step aside after a dismal debate performance last month.

Starmer praised Biden as he made a statement in the House of Commons about his attendance at the recent NATO summit in Washington and last week's European Political Community meeting in England.

The UK leader, elected earlier this month, said the US president was "a man who, during five decades of service, never lost touch with the concerns of working people and always put his country first".

He was "a true friend of the Labour movement" and "his presidency will leave a legacy that extends far beyond America to freedom and security on this continent".

"Most of all, of course, in our steadfast resolve to stand by the people of Ukraine. He leaves the Nato alliance stronger than it's been for decades," Starmer added.

Biden's decision not to seek re-election in a letter stunned the world and upended the 2024 race for the White House.

He endorsed Vice-President Kamala Harris as the Democratic Party's new nominee to take on Republican candidate Donald Trump.

During an event earlier on Monday, Starmer said it would have been "not an easy decision" for Biden to bow out of the contest.

"But a decision that I know that he will have arrived at taking into account the best interests of the American people, and I look forward to working with him for the remainder of his presidency," he added.

Starmer said he would work with whoever becomes US president after Biden's term ends in January 2025.