Sir Keir Starmer has said it would not have been an easy decision for Joe Biden to abandon his hopes of re-election to the White House.

The Prime Minister said he respected a decision made in the “best interests of the American people” and insisted he would work with whoever won the US election in November.

Mr Biden bowed out of the 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 in his place.

Sir Keir, who had talks at the White House with Mr Biden earlier this month while in Washington for the Nato summit, said: “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, 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.

“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.”

Sir Keir only met Mr Biden for the first time during the Nato summit and he is not thought to have ever met Ms Harris.

The 81-year-old 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”.

In a post on his social media account, he said: “My fellow Democrats, I have decided not to accept the nomination and to focus all my energies on my duties as President for the remainder of my term.

“My very first decision as the party nominee in 2020 was to pick Kamala Harris as my Vice President. And it’s been the best decision I’ve made. Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year. Democrats — it’s time to come together and beat Trump. Let’s do this.”

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 in his statement he would address the nation on the matter next week, adding: “For now, let me express my deepest gratitude to all those who have worked so hard to see me re-elected. I want to thank Vice President Kamala Harris for being an extraordinary partner in all this work. And let me express my heartfelt appreciation to the American people for the faith and trust you have placed in me.

“I believe today what I always have: that there is nothing America can’t do – when we do it together. We just have to remember we are the United States of America.”

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 Joe Biden’s “love for America and dedication to service”.

He said: “Working with Joe Biden, I saw firsthand his love for America and dedication to service.

“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”.