Last month, Parliament voted in favour of an amendment that supported most of the PM's deal but called for
backstop - which is a last-resort option to prevent a hard border in Ireland - to be replaced with "alternative arrangements". The prime minister is now in talks with Brussels to seek these changes to the backstop.
But if no changes are agreed by Wednesday, which is likely, Mrs May will address MPs and say more time is needed. She will then table a "neutral motion" for debate.
On Thursday, MPs will debate the motion - and potentially any amendments, such as Labour's - in a series of parliamentary votes.