There is "no alternative plan on the table" to Britain's Brexit deal negotiated with the European Union and a rejection by parliament risks sending the U.K. "back to square one with all the division and uncertainty that comes with it," Prime Minister Theresa May wrote in a column in The Sun newspaper.
May said the Northern Ireland "backstop" - a plan that involves keeping the U.K. within an EU customs union even after a 21-month planned transition period - would ensure there would be "no problems on the Irish border" even if not all arrangements are settled in time for the U.K.'s withdrawal.
The backstop is "an insurance policy" that her government does not expect to use and the U.K. will not become stuck with it, May said.
The "backstop" is the focal point of hostility among Brexiteers who say May's deal leaves the U.K. in perpetual limbo, unable to escape the bloc's influence. Several members of May's cabinet resigned on Friday amid a gathering plot to oust her from the leadership.
British Prime Minister Theresa May said on Sunday toppling her would risk delaying Brexit and she would not let talk of a leadership challenge distract her from a critical week of negotiations with Brussels.
In the days since she unveiled a draft EU divorce deal, May's premiership has been thrust into crisis. Several ministers, including her Brexit minister, have resigned and some of her lawmakers are seeking to oust her.
More than two years after the United Kingdom voted to leave the EU, it is still unclear how, on what terms or even if it will leave as planned on March 29, 2019.
May has vowed to fight on, but with both pro-EU and pro-Brexit lawmakers unhappy with the draft agreement, it is not clear she will be able to win the backing of parliament for it, raising the risk Britain leaves the EU without a deal.
"These next seven days are going to be critical, they are about the future of this country," May told Sky News. "I am not going to be distracted from the important job."
"A change of leadership at this point isn't going to make the negotiations any easier ... what it will do is mean that there is a risk that actually we delay the negotiations and that is a risk that Brexit gets delayed or frustrated."
To trigger a confidence vote, 48 of her Conservative lawmakers must submit a letter to the chairman of the party's so-called 1922 committee, Graham Brady.
More than 20 lawmakers have said publicly that they have done so, but others are thought to have submitted letters confidentially. Brady told BBC Radio on Sunday the 48 threshold had not yet been reached.
Brady said he thought it was likely May would win any confidence vote, making her immune to another challenge for 12 months under the party's rules.
Mark Francois, one lawmaker who has submitted a letter, said he expected some colleagues were taking soundings from local party members over the weekend before making a decision.
At the centre of concerns over the deal is the Northern Irish backstop, an insurance policy to avoid a return to border checks between the British province and EU-member Ireland.
Critics say it would leave Britain bound to the EU in perpetuity and risks dividing the United Kingdom by aligning Northern Ireland more closely with the EU's customs rules and production standards than mainland Britain.
The DUP, a small Northern Irish party which props up May's minority government, has threatened to pull its support if the backstop means the province is treated differently from the rest of the United Kingdom.
DUP deputy leader Nigel Dodds said on Sunday it was "time to work for a better deal which does not undermine the integrity of the United Kingdom".
May said negotiations were continuing and she intended to go to Brussels and meet European Commission President Jean-Claude Juncker. She said she would also be speaking to other EU leaders ahead of an EU summit to discuss the deal on Nov. 25.
"We won't agree the leaving part, the withdrawal agreement, until we have got what we want in the future relationship because these two go together. The focus this week will be on the future relationship," she told Sky.
"It is the future relationship that delivers on the Brexit vote."
Former foreign minister Boris Johnson, who resigned in July over May's Brexit plans, said it was "either a tragic illusion or an attempt at deception" to think issues with the exit deal could be remedied in the next stage of talks.
"I have heard it said that this is like a football match, in which we are one-nil down at half-time, but ... we can still pull it back and get the Brexit we want," Johnson wrote in his weekly column for Monday's Daily Telegraph.
"We are about to give the EU the right to veto our departure from the customs union. Why should they let us go?" British newspapers reported that five senior pro-Brexit ministers were working to pressure May to change the deal, but May said she saw no alternative plan on the table.
Former Brexit minister Dominic Raab, who resigned on Thursday in protest at the deal, said he supported May as leader but her deal was "fatally flawed" and she must change course.
"I still think a deal could be done," Raab told the BBC.
"The biggest risk of no deal is taking a bad deal to the House of Commons ... it is very important to take the action now."
Opposition Labour leader Jeremy Corbyn said his party would vote against May's deal and the government should go back to Brussels for further negotiations. He said that was a priority ahead of pushing for a second referendum on the final agreement.
"It's an option for the future, but it's not an option for today, because if we had a referendum tomorrow, what's it going to be on? What's the question going to be?" Corbyn told Sky News.