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.
The Speaker of the House, John Bercow, has picked eight options for MPs to vote on later.
These include proposals such as agreeing to leave on 12 April without a deal, forming a customs union and calling for a confirmatory referendum.
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
How are MPs likely to vote?
Conservative MPs will be given a free vote, meaning they will be able to support or reject any proposal without pressure from party whips. Cabinet ministers will be abstaining.
The decision followed warnings that more than a dozen ministers might quit if they were told they had to follow party orders.
Labour MPs are being whipped to support the party’s own proposal as well as a number of others, including one for a referendum to endorse any deal.
There was some confusion over the confirmatory public vote option. Labour MP Peter Kyle helped draw up the motion and told Today that he expected his leader Jeremy Corbyn to order his MPs to back it.
But his colleague Barry Gardiner cast doubt on the support from Labour’s frontbench, telling the programme the amendment makes it “look like a public vote is an attempt to Remain”, adding: “It is not where our policy has been.”
MPs later confirmed to the BBC they had been ordered to support the proposal.
The DUP’s leader in Westminster, Nigel Dodds, has co-signed two proposals – one asking for the result of the EU referendum in 2016 to be respected and another backing the Malthouse Compromise.
What is the PM’s next move?
Mrs May continues to try to win MPs round to her deal, which has been heavily rejected twice. She is expected to address the Conservative backbench 1922 Committee on Wednesday evening.
Ms Leadsom said there was a “real possibility” the PM’s deal could come back for a vote on Thursday or Friday, adding: “If we could simply get the withdrawal agreement bill under way…once we have done that, once we have left the EU, we can then look at what our future relationship will look like.”
And during Prime Minister’s Questions – after being told by her fellow Tory MP Andrew Bridgen that his constituents could not trust her to deliver Brexit – Mrs May said she could guarantee delivering on Brexit if “this week” MPs like him supported her deal.
Friday is the day written into law for the UK to leave the EU, but later MPs will vote on a statutory instrument to confirm a delay – with the earliest Brexit is likely to happen now being 12 April.
Writing in the Daily Mail, Mr Rees-Mogg – who is chairman of the pro-Brexit European Research Group of Tory MPs – said “an awkward reality needs to be faced” and he was ready to back the deal so long as it won DUP support.
Fellow ERG member, Tory MP Daniel Kawczynski, tweeted that he had appealed to members of the DUP to abstain from voting on the PM’s plan if they cannot back it, saying it could be enough to get the deal “across the finishing line”.
But ex-Foreign Secretary Boris Johnson told the BBC there was “no point” supporting Mrs May’s deal “without any sign the UK is going to change its approach in phase two” of the negotiations. Otherwise he said he feared the country would be indefinitely tied to the EU’s rules.
BBC political editor Laura Kuenssberg said the not-so-subtle subtext of Mr Johnson’s remarks was “if the PM promises to go soon, then she might get my vote”.
According to a copy of a business motion released by Labour’s Hilary Benn, there will be about five hours of debate on different options.
The Speaker will select around half a dozen options, with MPs marking on paper each option with a “yes” or “no”.
MPs will use both lobbies for completing the indicative vote ballots.
Voting by paper ballot will take place at about 19:00 GMT, with the results announced by Mr Bercow later that evening.
The process is likely to continue on Monday as MPs seek to whittle down options which could command majority support in Parliament.
The government has until 12 April to propose a different way forward to the EU if it cannot get the current agreement through Parliament.
