The announcement comes less than 24 hours after the Brexit Secretary David Davis and his deputy quit the government in protest over a Brexit compromise that was endorsed by the Cabinet on Friday.
A Downing Street spokesman said: “This afternoon, the Prime Minister accepted the resignation of Boris Johnson as Foreign Secretary. His replacement will be announced shortly. The Prime Minister thanks Boris for his work.”
His departure amid growing anger from Conservative Brexiteers increases the likelihood that Theresa May could be forced from office in a leadership challenge.
Mr Davis handed in his resignation late on Sunday night over the Prime Minsiter's plans to maintain a 'common rulebook' with the EU on goods after Brexit, effectively keeping one foot in the customs union and remaining tied to Brussels regulations and ruling out free trade deals with third countries.
Explaining his decision, Mr Davis said the UK was giving “too much away, too easily” in the Brexit talks, adding: “In my view, this policy has got a number of weaknesses."
The announcement came half an hour before the Prime Minister was due to address MPs on the Cabinet agreement reached at Chequers last week, and a few hours before a crucial appearance before the 1922 Committee of Conservative MPs which could decide her fate.
Scottish Conservative leader Ruth Davidson, a long-time rival of Mr Johnson's, made her first public comment on Monday's drama when she tweeted: "The Prime Minister is correct to accept the Foreign Secretary's resignation. Statement to follow PM's address to the House."
The SNP's Westminster leader Ian Blackford tweeted: "The foreign secretary should never have had the opportunity to resign. He should have been sacked months ago as a national embarrassment."
"This is complete and utter chaos. The country is at a standstill with a divided and shambolic Government. The Prime Minister can't deliver Brexit and has zero authority left."