Boris Johnson has insisted that he is "not bluffing" about delivering a no-deal Brexit on October 31.
The front runner to succeed Theresa May in little over a fortnight urged European leaders to "look deep into our eyes" and understand that the UK will leave the EU with or without an agreement on Halloween if he becomes Prime Minister.
The warning, in an interview with The Sunday Telegraph, will rankle in Brussels where EU figures insist they will not re-open the exit deal agreed with Theresa May.
Johnson hopes to secure changes by making clear that he will walk away if Brussels refuses to budge, after May failed to deliver on a similar promise in March.
Johnson's warning came as it emerged that pro-EU MPs, led by Dominic Grieve, the former Attorney-General, were planning an amendment to Government legislation in an attempt to prevent Johnson from thwarting parliamentary opposition to a no-deal Brexit.