Tory leadership rivals Boris Johnson and Jeremy Hunt are at loggerheads over how the UK should leave the EU.
Both contenders for prime minister claim they can renegotiate a Brexit deal that the EU says is closed.
Mr Johnson said the UK must leave on 31 October "deal or no deal" but Mr Hunt called this a "fake deadline" that could trigger a general election if Parliament rejects a no-deal Brexit.
The winner of the contest will take over from Theresa May on 24 July.
The two men will face more questions from the public on Wednesday in a digital hustings, streamed on the Conservative Party's Facebook and Twitter accounts.
