The PM says he tried to stay in office but blames “herd instinct” in Westminster for his departure.
Boris Johnson has said it is “painful not to be able to see through so many projects and ideas” as he announced his resignation as Tory party leader.
The PM was met with cheers from family and staff as he made his resignation speech outside No 10.
He said he had tried to persuade colleagues it would be “eccentric” to change leader and blamed the “herd instinct” in Westminster for his exit.
He said a timetable for a leadership contest would be announced next week.
Mr Johnson faced a mass revolt by ministers over his leadership, sparked by the dramatic resignation of Chancellor Rishi Sunak and Health Secretary Sajid Javid on Tuesday evening.
He resisted calls to quit – including from newly-appointed Chancellor Nadhim Zahawi – for 48 hours until it became clear that he had lost the confidence of his party and could not continue.
Speaking outside Downing Street, he said: “The reason I have fought so hard in the last few days to continue to deliver that mandate in person, was not just because I wanted to do so, because I felt it was my job, my duty, my obligation to you to do what we promised in 2019.”
“I regret not to have been successful in those arguments,” he said but added that: “In politics, no one is remotely indispensable.”
He said he was proud of his achievements in office citing taking the UK out of the EU, the government’s response to Covid and rolling out the vaccine programme.
He also said he had led “the West in standing up to Putin’s aggression in Ukraine”.