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.
A senior government official said a motion for an election would be put forward if MPs take the first steps towards passing legislation to block no deal this week.