Theresa May has said she will quit as Conservative leader on 7 June, paving the way for a contest to decide a new prime minister.In an emotional statement in Downing St, she said she had "done my best" to honour the 2016 EU referendum result.
It would remain a matter of "deep regret" that she had been unable to deliver Brexit, she added.
Mrs May said she will continue to serve as prime minister while a Conservative leadership contest takes place.
She will step down as Tory leader on 7 June and a leadership contest is due to begin the following week.
Johnson announced his intention to run for prime minister once May steps down. Others expected to run include Dominic Raab, Esther McVey and Matt Hancock.
Whoever assumes control still must resolve key issues around Brexit.The UK is set to leave the EU on October 31, but the country's political future has never looked more uncertain.