US President Joe Biden has announced that he will end his candidacy for re-election, saying, "It is in the best interest of my party and the country."

It comes four months before Americans go to the polls, upending the race for the White House.

It follows weeks of intense pressure from fellow Democrats after a faltering debate performance against Republican Donald Trump at the end of June.

In a letter posted to his social media account, 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 my duties as President for the remainder of my term."

He said in his statement that he would address the nation on the matter next week.

President Biden thanked his Vice President, Kamala Harris, saying she was an "extraordinary partner."

"And let me express my heartfelt appreciation for the American people for the faith and trust you have placed in me," his statement added.

"I believe today and 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."

Last week he returned to his home in Delaware after being diagnosed with Covid, but said on Friday he was looking forward to "getting back on the campaign trail next week."

He has previously said only the "Lord Almighty" could make him withdraw, but later said he would consider withdrawing if he had a health condition.