Prominent Brexiteer Peter Bone said a no-deal scenario was the only way to guarantee the United Kingdom actually leaves the EU.
She did not say whether she would try a second time to get the deal passed by parliament if defeated, and refused to hold a second referendum, which she said would be "divisive and disrespectful" to the voters of the 2016 vote.
European Union leaders have made it clear they are ready to give May all the reassurances she needs but the deal is not open to debate.
The MPs are set to vote on two amendments to the Finance Bill, which grants funds to the Treasury to spend on implementing no deal arrangements, on Tuesday.
However, she reiterated that the current withdrawal deal was the only possible one.
Lawmakers are due to vote on whether to accept May's exit deal in the week beginning January 14.
But parliamentary opposition to her deal remains fierce, with the main sticking point being the safety net "backstop" measure - which would guarantee no hard border is erected on the island of Ireland in the event that post-Brexit trade negotiations between the United Kingdom and the bloc prove unsuccessful.
"The backstop remains the poison which makes any vote for the Withdrawal Agreement so toxic". The MPs say that the move is meant to remove the threat of a no-deal Brexit by making clear no government could function effectively in such a scenario.
But she added: "If Parliament says 'This is just nonsense, this is no better than Theresa May's [deal],' we will put our hands up to that".
In a message also issued to her own party's Brexiteer MPs, Mrs May said: "We have got people who are promoting a second referendum in order to stop Brexit, and we have got people who want to see their flawless Brexit".
Following the publication of the cross-party letter by MPs opposed to a no-deal Brexit, eurosceptic Conservative backbencher Sir Bernard Jenkin, chairman of the Commons Constitutional Affairs Committee, told BBC Radio 4's Today programme: "Leaving the European Union is set in the law".
But she admitted she had no way of knowing what might happen if MPs rejected her deal, adding that the country would be in uncharted territory. "It was leave or remain, and the way you leave is to come out on March 29". The government, for its part, has forecast a potential economic slump of more than nine percent in the wake of a no-deal Brexit.
The survey of more than 25,000 voters also showed that 41 percent of Britons thought the final decision about Brexit should be made by a new public vote versus 36 percent who believe it should be up to parliament.
Theresa May this morning warned a second referendum would be "disrespecting" people who voted for Brexit.
But Mr Corbyn would be under pressure back another referendum if this proved impossible, with a YouGov poll suggesting support for another vote is high among Labour supporters.