MPs are preparing to vote for their preferred Brexit option, with the PM due to meet Tory backbenchers in an effort to win them over to her deal.
Some MPs want the PM to name the date she will leave No 10 to have any hope of getting their backing for her deal.
The BBC's Laura Kuenssberg said some leading Brexiteers were "tiptoeing their way" towards supporting her plan.
MPs are now voting on a business motion to confirm the way the debate will be carried out.
Conservative MP Jacob Rees-Mogg has said of the PM's deal: "Half a loaf is better than no bread."
He told BBC Radio 4's Today programme he did not "begin to pretend this is a good deal or a good choice", but he would support the PM's plan if it had the backing of the Democratic Unionist Party.
The 10 Northern Irish MPs are seen as the key to securing the deal, but they have urged Tory MPs to "stand firm" in their opposition unless there were "significant changes".
Commons leader Andrea Leadsom said the government was still in talks to persuade the DUP to back it, along with other MPs on their own benches.
Meanwhile, the President of the European Council Donald Tusk told MEPs they "cannot betray the six million people who signed the petition to revoke Article 50, the one million people who marched for a people's vote, or the increasing majority of people who want to remain in the EU".
He said the people "may feel that they are not sufficiently represented by the UK Parliament, but they must feel that they are represented by you... because they are Europeans".
Having voted to seize control of Commons business, backbench MPs will vote on Brexit alternatives later.
Conservative backbencher Sir Oliver Letwin, whose cross-party proposal ushered in today's debate, said the only way leaving the EU with no-deal can be prevented is by crystallising an alternative majority and trying to carry it forward.
He said that if MPs supported the prime minister's deal in another meaningful vote this would be "the easy route".
He said he "profoundly hopes" that if on Monday there is a majority view in favour of a particular position, that the government will say that it will carry that forward.
Timetable for Wednesday in Parliament
Now: MPs vote on the procedure for indicative votes
15:15: The Speaker announces which Brexit proposals he has chosen to be debated and voted on, and the debate begins
19:00: MPs vote with paper and pen for their preferences
19:30: Debate on statutory instrument (SI) bringing Brexit delay into law
21:00: Vote on SI
21:30: The Speaker announces the results of the indicative votes - though he could announce them earlier during SI debate
All times approx
Read also:Â Â Brexit: MPs vote to take control of Brexit process for indicative votes
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Source: bbc