The resignations of the two Cabinet "big beasts" comes just days after Mrs May secured senior ministers' agreement at Chequers for a Brexit plan about which both men had expressed reservations.
Mr Davis was first to go, announcing his exit just before midnight on Sunday.
But there was growing speculation about Mr Johnson's plans on Monday after he failed to attend a meeting of the Government's Cobra emergencies committee as well as a summit of Western Balkan nations being held in London.
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At 3pm on Monday, a statement was issued by Downing Street to say: "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."
Staunch Brexiteer Dominic Raab was named as Mr Davis's replacement as Secretary of State for Leaving the EU.