Johnson’s decision to leave the government came just hours after the resignation of the minister in charge of Brexit negotiations, David Davis.
It leaves her plan to for Britain’s future relationship with the European Union in tatters, just three days after it was agreed with her divided Cabinet.
It also puts her own political future under threat.
“This afternoon, the Prime Minister accepted the resignation of Boris Johnson as Foreign Secretary,” a statement from Downing Street said.
“His replacement will be announced shortly. The Prime Minister thanks Boris for his work.”
Davis resigned late on Sunday night, declaring he could not support May’s Brexit plan, it involved too close a relationship with the EU and gave only an illusion of control being returned to the UK after it left the EU.
“It seems to me we’re giving too much away, too easily, and that’s a dangerous strategy at this time,” Davis said in a BBC radio interview Monday morning.