Boris Johnson insists he's not bluffing about leaving the EU without a deal, if a new agreement isn't reached.
At least 20 Conservative MPs are reportedly planning to stop such a move if he becomes party leader.
Jeremy Hunt, his rival to replace Theresa May as British Prime Minister, says he'd take the country out in that manner with a "heavy heart".
Mr Johnson believes there would be serious consequences if the UK doesn't leave the EU.
He says the UK is facing an 'existential threat' and if the process goes beyond October 31st, the Conservative Party will never recover the trust of the public.
Ballot papers have been dispatched to around 160,000 Conservative Party members around the country to choose between Mr Johnson and Mr Hunt as the next leader - and the next prime minister.
The vote closes on 22 July, with the result announced the following day.