Theresa May will make a third attempt to get her EU withdrawal deal through Parliament in the next week.
She told MPs that if her deal fails again to get their backing, a lengthy delay to Brexit may be needed.
The prime minister’s warning comes ahead of a Commons vote later on whether to ask the EU for permission to delay Brexit beyond 29 March.
MPs voted on Wednesday evening to reject a no-deal Brexit under any circumstances.
No date has yet been set for the third so-called “meaningful vote”.
MPs will vote on a government motion which could see a delay to the UK’s departure from the EU.
The UK government said there could be a short delay until 30 June, if MPs approve Mrs May’s deal by 20 March – the day before the next EU summit in Brussels.
Alternatively, there could be a much longer one, requiring the UK to take part in elections for the European Parliament in May, the prime minister has said, in the event her deal is not approved.
However, any length of extension has to be agreed by the EU.
The DUP – which twice rejected Mrs May’s deal in the Commons – said it was due to have talks with the government on Thursday to see if a solution could be found allowing its MPs to support the PM in a future vote.
A party spokesperson said they wanted to find “a sensible deal for the entire UK and one that works for our neighbours in the Republic of Ireland”.