May, the leader of the Conservative party, believed she had secured a hard-won agreement with her deeply divided cabinet of ministers on Friday to keep the closest possible trading ties with the EU.
But as Foreign Secretary in May's government, he was prone to gaffes and criticized for not being on top of his brief.
Times columnist Rachel Sylvester said May´s authority "is utterly destroyed at the very moment she needs the credibility to assert herself in the negotiations with the EU".
Boris Johnson's resignation dominates Tuesday's United Kingdom front pages, with papers bitterly divided over the former foreign secretary's dramatic exit from cabinet, and what it means for Theresa May.More news: Arsenal complete move for Sampdoria midfielder Lucas Torreira
The UK government now appears to be moving in the right direction on Brexit and the European Union should be "similarly pragmatic and fair", according to Airbus chief executive Tom Enders.
Mr Johnson does not pull any punches, saying Brexit "should be about opportunity and hope" and a "chance to do things differently", but "that dream is dying, suffocated by needless self-doubt".
Jeremy Hunt, who has replaced Mr Johnson as foreign secretary, said he would be "four square" behind her. "The Prime Minister thanks Boris for his work".
The man leading the UK's negotiating team, David Davis, resigned late on Sunday night, saying that he did not agree with the UK's proposals, so was the wrong person to be going into negotiations with them. The group of Tories Rees-Mogg speaks for won't support the Brexit plan as it stands, but Labour lawmakers said it wasn't yet close enough to their position to win their support.More news: Carmelo Anthony to Rockets? Juicy Summer League sighting blows race wide open
Behind the scenes, European Union member states are fearful a collapse of Britain's minority Conservative Government would spell the end for Brexit negotiations and lead to a no deal.
"What the Prime Minister needs to do is to give up on the Chequers proposal. which does not actually deliver Brexit", he told LBC Radio.
She was not left in the clear by her MPs either, with challenges from hardline Brexiteers - with Peter Bone claiming campaigners had been "betrayed".
May´s Conservative opponents could trigger a confidence vote against her if at least 48 MPs support it, but to actually force her from office 159 MPs would have to vote against her - a figure hardliners may not be able to reach.More news: West Ham signs Wilshere from Arsenal
Long before the Brexit referendum and its aftermath, the Conservative Party was split on Europe, and May has struggled to unite the warring wings under her leadership.
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