Joe Biden’s presidency “will leave a legacy that extends far beyond America”, Sir Keir Starmer said after the US president announced he would not seek a second term in the White House.

The Prime Minister said the 81-year-old’s decision to abandon his re-election campaign would not have been an easy one.

Sir Keir 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 told MPs: “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.”

Mr Biden bowed out of the presidential race in response to mounting pressure about his age and his inability to take on Republican rival Donald Trump.

The US president backed vice president Kamala Harris to be the Democrat nominee.

Sir Keir, who had talks at the White House with Mr Biden earlier this month while in Washington for the Nato summit, earlier told reporters at the Farnborough International Airshow: “I respect that decision that he has now made.

“Not an easy decision, 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 declined to comment on his relationship with Ms Harris – who he has never met – but insisted the UK would work with whoever became president.

“Obviously in the first instance, it’s for the Democratic Party to decide who they want to put forward. It is then for the American people to decide who they want as their president,” he said.

“My approach will be to respect that decision-making and to be clear that we will work with whoever the American people elect into office, as you would expect, particularly given the nature of the special relationship between our two countries, forged in difficult circumstances, endured for years, and very important to me and very important to all American presidents.”

The US president announced on Sunday he would not seek another term in office, saying it was “in the best interest of my party and the country for me to stand down”.

He said it had been the greatest honour of his life to serve as president.

“And while it has been my intention to seek re-election, I believe it is in the best interest of my party and the country for me to stand down and to focus solely on fulfilling the duties as president for the remainder of my term,” he said.

The decision comes after escalating pressure from Mr Biden’s Democratic allies to step aside following his faltering June 27 debate.

Mr Biden’s term in office ends at noon on January 20 2025.

Vice president Ms Harris said she was “honoured” to have the endorsement of Mr Biden.

In a statement, she thanked the US president for his “extraordinary leadership” and for making a “selfless and patriotic act” by stepping aside.

She said: “I am honoured to have the president’s endorsement and my intention is to earn and win this nomination.”

Former prime minister Rishi Sunak praised Mr Biden’s “love for America and dedication to service”.

Mr Sunak said: “Our partnership has led to significant achievements, including Aukus, steadfast support for Israel and joint efforts in defending our people from Houthi threats.

“I wish him all the best.”

The US ambassador to the UK Jane Hartley said: “In every endeavour, President Biden has demonstrated a profound sense of duty and an unwavering commitment to the common good.

“His decision not to pursue another term is a selfless act that highlights his dedication to our country.”

Reform UK leader Nigel Farage said: “I predicted this would happen in September 2023.

“Whoever they pick, Trump will win in November.”

Writing on his Truth Social platform, Mr Trump said: “Joe Biden was not fit to run for President, and is certainly not fit to serve – And never was!”

He added: “We will suffer greatly because of his presidency, but we will remedy the damage he has done very quickly. MAKE AMERICA GREAT AGAIN!”

Former president Barack Obama praised Mr Biden as a “patriot of the highest order”, saying the US president had put American interests ahead of his own as he decided not to seek re-election.

But he stopped short of endorsing Ms Harris as other senior Democrats have.

He said: “I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges.”

Ukrainian President Volodymyr Zelensky said his country was “grateful to President Biden for his unwavering support”.