The Conservative leader's statement to parliament to explain the terms of the divorce deal with the European Union was met with a torrent of opposition.
MPs from all parties lined up to say they would reject it, but the attacks on her own side were particularly brutal.
One Conservative MP, Andrew Bridgen, directly asked her to resign. Another, Jacob Rees-Mogg, threatened to submit a letter of no confidence in her leadership -- and did so shortly after the session.
May conceded that Brexit required "difficult choices" but insisted the deal she had agreed after months of negotiations was in "the national interest".
She paid tribute to the work of her Brexit Secretary Dominic Raab, one of two cabinet ministers who earlier quit over the deal.
He was followed by several junior colleagues, prompting speculation the entire government could collapse.
May said she "respects their views".
'Chaos'
Opposition Labour leader Jeremy Corbyn told parliament: "The government is in chaos."
He outlined his critique of the Brexit deal as Conservative MPs behind May sat in silence, many checking their phones.
Nigel Dodds of the Democratic Unionist Party (DUP), the Northern Irish party on whom May relies for her majority, also pulled no punches in his warning that Britain must not become a "vassal state" of the EU.
"I could today stand here and take the prime minister through the list of promises and pledges she made to this House and to us, privately, about the future of Northern Ireland in the future relationship with the EU," he said.
"But I fear it would be a waste of time since she clearly doesn't listen."
'Not a coup'
Rees-Mogg, leader of the powerful European Research Group (ERG) of Brexiteer Conservative MPs, requested May's advice on whether he should try to unseat her -- before shortly after, announcing he would.
He said he did not know whether his action brought closer the possibility of a leadership contest, which requires the submission of 48 letters of no confidence.
But he said: "There is absolutely no support for this deal."
Rees-Mogg made his announcement after meeting with eurosceptic colleagues, including former foreign secretary Boris Johnson, in a committee room of parliament while May continued to speak in the Commons chamber.
"It's like a bloody alternative government," muttered an angry minister as he walked past, warning the eurosceptics were doing nothing to help deliver Brexit.
Later, addressing a hastily-convened press conference on the steps of parliament, Rees-Mogg was forced to deny he was leading a coup.
"This is working through the procedures of the Conservative party," he insisted, as a protester yelled "Stop Brexit" at him through a loudspeaker.
Pro- and anti-Brexit demonstrators jostled for pavement space outside parliament, waving placards reading "Save Brexit"; "Leave means leave" and "Stop the Brexit mess!"; "We want a people's vote".
Strongly held views
In her attempts to persuade MPs to back the agreement, May warned eurosceptics that rejecting it could lead to "no Brexit at all".
Aides said that referred to the possibility of a snap election in which Labour took office and called a second referendum on Brexit.
But pro-European MPs in the chamber were so vocally enthusiastic about this idea that she was forced to play it down.
"We are making no plans for no Brexit because this government is going to deliver on the vote of the British people," May said.
Pro-EU MP Anna Soubry hailed May's "absolute commitment and dedication" but called for another Brexit vote, saying: "This is not the promised deal."
There were some voices backing the prime minister, with Commons leader Andrea Leadsom -- among the cabinet ministers earlier tipped to resign -- saying she was "determined to support her".
May's spokesman acknowledged "there are strongly held views" about Brexit, but said she would fight any attempt to remove her.
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