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.

Following the announcement on Sunday evening, President Biden gave his endorsement in the presidential race to Vice President Kamala Harris.

In his endorsement, he said his “first decision as the party nominee in 2020” was to select Ms Harris, and said “it’s been the best decision I’ve made”.

Kamala Harris said she would do "everything in my power to unite the Democratic Party, and unite our nation to defeat Donald Trump".

In a letter posted to his social media account announcing his resignation, he said it had been the greatest honour of his life to serve as president.

LIVE UPDATES: Joe Biden withdraws from US presidential race

"And while it has been my intention to seek reelection, 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 wrote in his statement.

Calls for Biden to withdraw from the race began to grow after a disastrous debate performance in late June against Donald Trump.

During the debate he was criticised for often being incoherent and speaking in a weak voice - something which his party claimed as down to a cold.

Following it, there were growing calls from within his own party for him to stand aside, with a growing number of congressional Democrats worried his campaign would hurt their own re-election chances in November.

A handful of senators, who sit in the upper chamber of Congress, also called for him to withdraw.

Less than two weeks ago, President Biden hosted a summit with Nato leaders, knowing he would face intense scrutiny following his debate performance.

The occasion did little to calm nerves within his own party, with him mistakenly calling Ukrainian President Volodymyr Zelensky as “President Putin”, and appearing to refer to the vice president as "Vice President Trump".

While Kamala Harris has the backing of the current president, it is unclear at the moment if she will be selected unopposed as the new candidate.

Along with the president, she already has the backing of former President Bill Clinton and his wife and former presidential candidate Hillary Clinton.

Barack Obama, who Joe Biden served under as vice president, released a statement praising Mr Biden as "one of America’s most consequential presidents" - but did not mention Kamala Harris.

Following his withdrawal on Sunday evening, the BBC’s US partner network CBS reported the Democratic National Committee was holding an emergency meeting.

The focus will now be on the Democratic National Convention which takes place next month in Chicago.

Biden swept the primaries, meaning that the delegates representing each state at the convention were pledged to vote for him - although they will now presumably be released to vote for another candidate.