MPs have set out details of their plan to consider other Brexit options, as Theresa May was warned more ministers could quit unless she changes course.
The Commons will begin voting on alternatives on Wednesday, in a process likely to continue into next week.
MPs will fill out a series of ballots testing support for different ideas.
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 feared the country would be indefinitely tied to the EU's rules.
MPs took the unprecedented step of voting to seize control of the parliamentary timetable on Monday, in an attempt to end the deadlock over the terms of the UK's exit.
Groups are now putting forward a variety of different options for the UK's future relationship with the EU.
Several of these are based on the assumption that the existing withdrawal agreement with the EU will be approved, albeit with changes to the controversial Northern Ireland backstop.
Others call for a basic free trade agreement with the EU and another referendum on whether Brexit goes ahead. It will be up to Commons Speaker John Bercow to decide what is voted on.
What are some of the options?
Customs union: This calls for the UK to negotiate a new customs union with the EU immediately after it leaves.
Common Market 2.0:The UK would remain in the single market by rejoining the European Free Trade Association (EFTA) and staying in the European Economic Area (EEA). A "comprehensive customs partnership" would replace the Irish border backstop plan. It would accept continued freedom of movement but with conditions.
EFTA and EEA: The UK would rejoin EFTA and sign up to existing EEA rules and obligations but make them enforceable through the UK courts. Rejects any customs union with the EU, instead seeking agreement on new arrangements for Northern Ireland.
Malthouse compromise plan A: Mrs May's withdrawal deal but without the backstop, which would be replaced by alternative arrangements.
Another referendum: The public would vote on any Brexit deal which is passed by Parliament before it is ratified.
Revoke Article 50: If the government has not passed its withdrawal deal, MPs would vote on a no-deal Brexit two days before the UK's leaving date. If MPs reject no deal, the prime minister would have to cancel Brexit altogether.