After a day when the eurosceptic foreign secretary cancelled meetings for crisis talks at his official residence in central London, Johnson made a decision to walk from his job - just hours after May's Brexit minister David Davis did the same.
Davis resigned late Sunday, saying May's plan to maintain close trade and regulatory ties with the European Union gave "too much away, too easily".
There was no immediate statement from Johnson, another loud pro-Brexit voice within May's divided government.
Davis and his deputy quit just two days after May announced she had finally united her quarrelsome government behind a plan for a divorce deal with the EU.
Monday brings little to the field on the economic calendar for the GBP, with only a speech due at 07:50 GMT from the BoE's MPC member Broadbent, who will be delivering the opening remarks at the BoE-hosted Economics and Psychology conference.
But on the other side of the party divide, they feel that her words have not been matched by her deeds, proposing to negotiate a deal which could leave Britain still accepting European Union rules and regulations without being able to influence them. "The Prime Minister thanks Boris for his work".
But if she can't force a compromise through her party that itself took months to stitch together, the prime minister's authority would be significantly diminished. May replaced him with another Brexit campaigner, junior minister Dominic Raab.
May's official spokesman, James Slack, says "we need to move forward at pace in these negotiations, and that's what we are going to do".
He added that plans to create a common rulebook for goods and agrifoods "will make it much more hard to do free trade deals" - a key goal of the Brexit supporters.
Some Brexit-supporting lawmakers are angry at the proposals, saying they will keep Britain tethered to the bloc and unable to change its rules to strike new trade deals around the world.
Davis said he did not want his resignation to become a rallying cry for May's ouster.
"I like Theresa May, I think she's a good prime minister, " Davis said.
The Brexit secretary became the fifth cabinet minister to step down in under a year after Amber Rudd, Michael Fallon, Priti Patel and Damian Green vacated their positions.
But a well-connected source has just told me it could be more serious than that.
"I'd have to be the champion of the policy that I didn't believe in. Others don't have that same responsibility".
Theresa May faces a mortal threat to her leadership of the Conservative Party and Government today. Jacob Rees-Mogg, who leads a bloc of Brexit-supporting Conservative MPs in Parliament, urged May to give up on the plan.
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