Prime Minister Theresa May has announced she will make a third attempt to get her Brexit deal passed by 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.
There have been some suggestions that Brexit could be delayed until 2021 if a deal is not reached.
The PM’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 later today 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 to Brexit, or a much longer one, depending on whether MPs backed the prime minister's existing withdrawal deal by 20 March - the day before the next EU summit.
If MPs approve Mrs May's deal before that summit in Brussels, then the extension would be until 30 June.
However, the PM warned that if the deal - which has twice been rejected by overwhelming majorities - is not approved, a longer extension will be needed, requiring the UK to take part in elections for the European Parliament in May.
"I do not think that would be the right outcome," said Mrs May. "But the House needs to face up to the consequences of the decisions it has taken.”