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”.
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.”
In a night of high drama on Wednesday, the Commons first voted on an amendment to reject the UK exiting the EU without a deal under any circumstances, by a margin of four.
That meant the government’s original motion – which had stated that the UK should not leave the EU without a deal on 29 March – was changed at the last minute.
The government had wanted to keep control of the Brexit process, and keep no-deal on the table, so they then ordered Conservative MPs to vote against their own motion.
That tactic failed. Government ministers defied those orders and there were claims Mrs May had lost control of her party.
The updated motion, to reject a no-deal Brexit under any circumstances, was passed by 321 to 278, a majority of 43.