Theresa May has vowed to fight on despite a series of resignations by Conservative colleagues determined to block her proposed Brexit deal.
She said: “Am I going to see this through? Yes.”
The Prime Minister even compared herself to former England cricketer Geoff Boycott, saying he “stuck to it, and he got the runs in the end.”
But Mrs May could face a vote of no-confidence by Tory MPs after pro-Brexit backbenchers called on her to go.
Brexit Secretary Dominic Raab resigned from the Cabinet, soon followed by Work and Pensions Secretary Esther McVey.
In his letter to the Prime Minister, Mr Raab said the deal represented a "very real threat to the integrity of the United Kingdom" because of provisions for Northern Ireland.
He also said he could not accept "an indefinite backstop arrangement" for the Irish border.
He said: "No democratic nation has ever signed up to be bound by such an extensive regime, imposed externally without any democratic control over the laws to be applied, nor the ability to decide to exit the arrangement."
There were also resignations from junior ministers and government aides.
Mrs May received a hostile reception as she delivered a statement to the House of Commons defending her proposed deal.
Labour leader Jeremy Corbyn said she had produced “a botched deal” and his party would vote against it.
But the Prime Minister was also repeatedly criticised by her own Conservative colleagues, with high-profile Tory MP Jacob Rees-Mogg suggesting to her face that she should be fired.
Mr Rees-Mogg later announced that he had written to Sir Graham Brady, the chairman of the 1922 Committee of backbench MPs, to say he had no confidence in Mrs May.
Under Conservative Party rules, if Sir Graham receives 48 letters then a ballot of Tory MPs will be held to decide whether to replace Mrs May as party leader.
Mr Rees-Mogg said: “I think there are many people in the Conservative Party, not just in Parliament but in the country at large, who feel that her service now should come to an end.
“She is a very dutiful person, she has served the country to the best of her ability but she has let us down in this deal.”
In a press conference later in the day, Mrs May made it clear she planned to continue as Prime Minister, and to keep pushing her proposed Brexit deal.
She even compared herself to former England cricketer Geoff Boycott, saying he “stuck to it, and he got the runs in the end.”
Mrs May said: “This deal delivers what people voted for and it is in the national interest”.
It would protect manufacturing, protect our security and protect the integrity of the United Kingdom, she said.
But Mrs May insisted that if the deal is rejected “it will be to take a path of deep and grave uncertainty when the British people just want us to get on with it.”
There are Conservative MPs who continue to support Mrs May. They include Northern Ireland Secretary Karen Bradley, who urged political colleagues to “get behind the Prime Minister”. She said: “This is a woman who has gone out and negotiated hard.”
But many Tory MPs were predicting last night that Mrs May would face a no-confidence vote. It’s possible that she could win any vote that takes place, allowing her to continue in office.
A particular concern for critics of the Brexit deal is the “backstop” arrangement designed to prevent a hard border with Northern Ireland.
Mrs May’s deal involves a temporary single custom territory, effectively keeping the whole of the UK in the EU customs union - until both the EU and UK agree that it is no longer necessary.
This would only come into effect if another solution to the border issue cannot be found by December 2020.
But some Brexiteers fear it means the UK would continue to follow EU rules without having any say over them.
The deal is fiercely opposed by the DUP, which has been propping up Theresa May’s government in the House of Commons.
DUP Westminster leader Nigel Dodds said: “I could today stand here and take the Prime Minister through the list of promises and pledges that she made to this House and to us privately about the future of Northern Ireland and the future relationship with the EU but I fear it would be a waste of time since she clearly doesn’t listen.”
Mrs May has depended on support from the DUP to win key Commons votes following the 2017 general election, when the Conservatives lost their majority.
The scale of the opposition the deal suggests Mrs May will find it extremely difficult to win Parliamentary backing for the proposals.
Under the EU Withdrawal Act, which came into law in June this year, the government is barred from signing a Brexit deal with the EU unless it has been approved by a vote in the House of Commons.