The government is expected to table a motion to hold a general election on 14 October if it is defeated by MPs opposed to a no-deal Brexit on Tuesday.
Boris Johnson said he did not want an election, but progress with the EU would be “impossible” if they won.
Tory rebels are joining forces with Labour to bring a bill designed to stop the UK leaving the EU on 31 October without an agreement.
It would force the PM to request a delay to 31 January 2020 in that event.
Speaking outside No 10, Mr Johnson insisted that with MPs’ backing, he would be able to achieve changes to the UK’s current Brexit deal – negotiated by Theresa May and rejected three times in the Commons – at an EU summit on 17 October.
But he said if MPs voted to block no deal they would “plainly chop the legs out from under the UK position”.