The 81-year-old president's decision comes four months before Americans go to the polls on November 5. 

Biden's decision follows weeks of intense pressure from fellow Democrats after a disastrous presidential debate performance against his Republican rival and former US president 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 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".