On Monday, May infuriated Conservative lawmakers who want to keep the closest possible ties with the European Union when she chose to accept a number of demands by hardline pro-Brexit MPs from her party.
Johnson, who led the main Brexit campaign in the 2016 referendum, resigned this month over May's strategy, triggering the government's biggest crisis since she lost her parliamentary majority after calling a snap election past year. These negotiations should be completed before the end of October to give the parliaments of both sides enough time to ratify the agreed document.
Johnson added that a "needless fog of self-doubt" had descended over the past 18 months but still praised Prime Minister Theresa May for her speech at Lancaster House on free trade.
For someone who less than a fortnight ago signed up to the government's Chequers deal, even toasting the PM over dinner, that's quite some feat.
A bristling Mrs May insisted at "absolutely no point" had that happened because "Brexit continues to mean Brexit".
However, while European Union leaders have made no secret of being ready to extend the deadline for a few weeks, there are deep reservations about any longer delay on Brexit, short of a revolutionary U-turn in Britain and a clear call from London to call Brexit off. His remarks are nonetheless a headache for the British Conservative leader who is trying to rally support for her Brexit plan, which would see Britain remain closely aligned with the European Union on trade rules for goods (though not services).
He said the vision set out by Mrs May in her Lancaster House speech in January 2017 for a "strong independent, self-governing Britain" had never been turned into a firm negotiating position.
'If 48 such letters are sent a vote of no confidence in the Prime Minister amongst Conservative MPs will take place.
May also sought to rally support in later meetings with Conservative Party backbenchers known as the 1922 Committee and with the Parliamentary Liaison Committee that includes the leaders of all the House of Commons select committees.
If a vote is triggered, the prime minister would need to win the votes of more than half of the Tories' 316 MPs to survive.
"If you look at the turbulence on the United Kingdom side, if you take a look at how much we still have to do to bridge gaps and arrive at something that is workable", one said, "it's only normal that people tend to think a bit more about preparedness". I thought it was the right vision then.
The resignation of his predecessor David Davis and others, and May's battles in parliament with pro- and anti-Brexit wings of her own Conservative party, have fuelled new questions in Brussels over whether London is capable of agreeing any deal this year to avoid chaos when it leaves in March.