Boris Johnson has insisted he does not want a general election, as he urged MPs not to vote for “another pointless delay” to Brexit.
But the BBC understands the PM would seek to call a vote, expected to be on 14 October, if MPs block no deal.
Tory ex-ministers are joining forces with Labour to stop the UK leaving the EU on 31 October without an agreement.
Speaking outside No 10, Mr Johnson said such a move would make further talks with the EU on a new deal “impossible”.
With MPs’ backing, though, he said he would be able to achieve changes to the UK’s current deal – negotiated by Theresa May and rejected three times in the Commons – at an EU summit on 17 October.
The chances of reaching a new agreement had been “rising” because the UK was preparing to leave the EU “come what may” by the latest deadline, he added.