The leading Brexiter said that he tried to support the line agreed at Chequers on Friday but while the “government now has a song to sing” he could not manage to support the plan agreed.
“The trouble is that I have practised the words over the weekend and find that they stick in the throat,” Johnson wrote. “Since I cannot in all conscience champion these proposals, I have sadly concluded that I must go.”
Johnson was the third minister to quit in 24 hours following the Chequers deal, although his resignation was announced by Downing Street at 3pm before he had a chance to complete his letter.
He then chose to release his own resignation letter before Downing Street had a chance to reply, breaking the usual convention that a minister’s resignation letter is released at the same time as the prime minister’s response.
May hammered out a compromise with her deeply divided cabinet in an all-day meeting at Chequers on Friday but Johnson decided he could not promote the deal after consulting friends and allies.
Johnson wrote that he believed May’s new plan amounted to “a semi-Brexit” with large parts of the economy “locked in the EU system, but with no UK control over that system”.
May’s negotiating pitch to the European Union would see the UK agree to adopt a “common rule book” on standards for food and goods in return for achieving greater divergence on services and digital regulation, a proposal that has worried hardline Brexiters since it was leaked last week.
Pressure on the foreign secretary had been mounting since fellow pro-Brexiter David Davis resigned as Brexit secretary on Sunday night, swiftly followed by his No 2 at the Department for Exiting the EU, Steve Baker.
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.”