By Laura Smith-Spark and Euan McKirdy, CNN

(CNN) -- UK Prime Minister Theresa May was plunged into a deep political crisis on Thursday as she was hit by a wave of resignations over her Brexit plan.

Two Cabinet ministers, two junior ministers and two parliamentary aides had quit by the time May stepped into the House of Commons to present the draft agreement approved by her Cabinet late on Wednesday.

As May attempted to sell her deal to fractious lawmakers, it was unclear if she could even hold her government together. But later on Thursday, the PM vowed in a news conference to fight for her Brexit deal.

"Serving in high office is an honor and privilege," she told a room of reporters.

"It is also a heavy responsibility -- that is true at any time but especially when the stakes are so high. I believe with every fiber of my being that the course I have set out is the right one for our country and all our people."

Brexit Secretary Dominic Raab, who has been leading Brexit negotiations with the EU, was the first Cabinet minister to quit, followed quickly by Brexit-backing Work and Pensions Secretary Esther McVey. Two junior ministers, two ministerial aides and the Conservative party's vice chairman also resigned.

Raab, the second Brexit Secretary to quit in four months, said on Twitter he could not "in good conscience" support the deal.

In the House of Commons, May urged MPs to back her plan. "Voting against a deal would take us all back to square one," she said. "It would mean more uncertainty, more division and a failure to deliver on the decision of the British people that we should leave the EU. If we get behind a deal we can bring our country back together."

May, who faces the uphill battle of winning Parliament's approval for the agreement, said she respected the views of those who had quit Thursday. She acknowledged that the Brexit process was "uncomfortable" and involved "difficult choices."

Opposition Labour Party leader Jeremy Corbyn slammed what he called a "botched" and "half-baked" deal, saying it "represents a huge and damaging failure" on the part of a government "in chaos."

"The government simply cannot put to Parliament this half-baked deal that both the Brexit Secretary and his predecessor have rejected," he said.

Nigel Dodds, deputy leader of Northern Ireland's Democratic Unionist Party -- which May relies on to prop up her minority government -- was also damning in his assessment of the deal, saying the Prime Minister "clearly doesn't listen."

Meanwhile, Conservative MP Jacob Rees-Mogg, who heads the influential pro-Brexit European Research Group, openly asked May why he should not demand her resignation.

He said May had pledged that the UK would leave the Customs Union, that she would maintain the integrity of the UK and that it would leave the jurisdiction of the European Court of Justice -- but that the deal did none of those things.

May had barely left the chamber following three hours of grueling questions before Rees-Mogg announced publicly that he was submitting a letter of no-confidence in May -- essentially an open call for the Prime Minister's own party to unseat her.

If 48 such letters are sent to Graham Brady, the party official who represents the interests of Conservative MPs, a leadership contest would be triggered.

Rees-Mogg wrote that the deal "has turned out to be worse than anticipated and fails to meet the promises given to the nation by the Prime Minister, either on her own account or on behalf of us all in the Conservative Party manifesto," according to the Press Association.

Perilous development

The day's drama began with Raab's unexpected resignation. Despite being a leading Brexiter he was thought to have been on board with May's plan.

His move triggered a series of resignations that imperil May's government. If more Cabinet ministers resign, it would make it hard for May to hold her administration together.

In her resignation letter, McVey said the draft agreement "does not honour the referendum" and that she could neither defend it nor vote for it.

Suella Braverman, another staunch Brexiter who resigned as a minister, tweeted that she found herself "unable to sincerely support the deal" and that the concessions made "do not respect the will of the people."

The Conservative Party's vice chairman, Rehman Chishti, also announced his resignation Thursday, citing May's performance in the House of Commons as one of the reasons. "The UK in effect will be part of a system where it will be a rule taker without any say on the rules," he said.

Former UK Independence Party leader Nigel Farage, who was a driving force behind Brexit, congratulated the former Brexit Secretary on his resignation, tweeting: "Well done Dominic Raab, a few more and we will be rid of this duplicitous Prime Minister."

Raab's predecessor, David Davis, quit over the so-called Chequers Agreement in July. Other high-profile resignations include former Foreign Secretary Boris Johnson and his brother, Jo Johnson, a junior transport minister.

May: 'Difficult choices'

Standing outside Downing Street on Wednesday following the five-hour Cabinet meeting, May declared she had won the backing of her senior ministers. "This is a decisive step which enables us to move on and finalize the deal in the days ahead," she said.

She also spelled out the "difficult choices" the government faced. "When you strip away the detail the choice before us is clear," she said. "This deal, which delivers on the vote of the referendum, which brings us back control of our money, laws and borders, ends free movement, protects jobs, security and our union, or leave with no deal, or no Brexit at all."

Thursday's front pages reflected the difficulties ahead. The Financial Times predicted a "backlash" following the "ferocious Brexit battle," while the Daily Mail focused on the Prime Minister's defiant tone with the headline, "I stand to fight."

The Guardian, which had supported Remain, pointed to the divisions that May's Brexit plan had created under the banner "a split cabinet, a split party and a split nation."

All sides 'still have a long road ahead'

The EU's chief negotiator Michel Barnier on Thursday called the draft agreement a "very important moment. What we have agreed... is fair and balanced, takes into account the UK's positions, (and) organizes withdrawal in orderly fashion and ensure no hard border... and lays the ground for an ambitious new partnership.

"Our work is not finished, we still have a long road ahead of us on both sides. We have no time to lose," he said, before handing the 558-page draft document to Tusk, who, in a tongue-in-cheek exchange with a reporter, said he had read "almost everything" in the document.

"I don't share the (British) Prime Minister's enthusiasm about Brexit as such. Since the very beginning, we have had no doubt that Brexit is a lose-lose situation and negotiations are only about damage control," said Tusk.

He said the agreement was now being analyzed by all member states and by the end of the week the EU 27 ambassadors would meet to share their views on it.

The commission intends to agree the framework by Tuesday, Tusk said, and a European Council meeting to "finalize and formalize the agreement" will be held on November 25.

Antonio Tajani, the president of the European Parliament, welcomed the draft Brexit deal, but warned it was only "the first step on a long road."

French PM Edouard Philippe warned that Britain could still crash out of the EU without a deal. "Nothing allows us to say at this stage that the agreement will ultimately be adopted. We must prepare ourselves for a potential outcome, which we do not want but which is on the table, that of an exit without an agreement," Philippe said Thursday.

German Foreign Minister Heiko Maas tweeted that he was ''very pleased'' the EU and the British negotiators had reached a draft Brexit deal, even as he pointed out that Europe had not yet approved it.