May and her cabinet team spent Friday at Chequers, the prime minister's country house, discussing Brexit policy.
"I don't think it's central to Foreign Secretary, but there you are".
Johnson, who favours a stronger break with the European Union, said in his resignation letter that Britain is "truly headed for the status of [EU] colony".
British foreign minister Boris Johnson has resigned, following former-Brexit minister David Davis out of the cabinet just days after Prime Minister Theresa May secured a hard-won agreement from senior ministers on an European Union exit strategy. Steve Baker, a junior Brexit minister, resigned along with Davis.
First, David Davis resigned as Brexit secretary.
The tousle-headed blond Johnson is one of Britain's best-known politicians, and one of the most prominent advocates for Brexit.
The likelihood of a Conservative leadership contest now looms.
Mrs May is expected to tell MPs that the strategy agreed on at Chequers is the "right Brexit" for Britain.
Johnson said that at a meeting of the cabinet to decide the plan on Friday, he had accepted that "my side of the argument were too few to prevail".
'The trouble is that I have practised the words over the weekend and find that they stick in the throat'.
Instead May was tonight scrambling to shore up her ailing Premiership amid fierce rumours she is set to face a no confidence vote, possibly within hours.
Labour leader Jeremy Corbyn said Mrs May should make way for a Labour administration if her Government can not get its act together quickly.
Hunt, 51, had served as health minister for more than 5-1/2 years, the longest term of any in the history of the state-funded National Health Service, weathering doctors' strikes, public discontent with funding levels and other challenges.
Labour MP Andrew Adonis said Raab's appointment could spell the "death" of the Tory leader, while writer and editor Hitcham Yezza said Brexit has now become even more "shambolic".
There was no immediate statement from Johnson, another loud pro-Brexit voice within May's divided government.