The Labour leader says he has "hard evidence" disproving claims there will be no border in the Irish Sea.
The former PM encourages voters to back three independent candidates running against his own party.
The hour-long BBC election debate comes less than a week before voters go to the polls.
Labour defends its plan to hold a Brexit referendum and extend the franchise, called a "fiddle" by Boris Johnson.
The party says it would set up a new agency to offer support and advice to smaller companies.
Email us at firstname.lastname@example.org
Send an SMS or MMS to +44 7624 800100
Follow Have Your Say on Twitter
Why you can trust BBC News