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.
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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.
However, sources suggested the Northern Ireland Bill could be pulled by ministers to avoid a separate amendment on abortion.
Johnson's assurance on his seriousness about delivering a no-deal Brexit came as 36 pro-Leave MPs — a quarter more than the number that voted against May's deal — signed up to StandUp4Brexit, a public campaign insisting the UK must leave the EU by October 31.
The pledges are a reprisal of a major grassroots campaign that previously foreshadowed the defeat of May's deal with the EU in the Commons, and highlight the pressure on Johnson to deliver on his pledge.
Asked whether he was bluffing over his insistence on leaving on October 31 with or without a deal, Johnson said: "We were pretty much ready on March 29. And we will be ready by October 31. And it's vital that our partners see that.
"They have to look deep into our eyes and think, 'my God, these Brits actually are going to leave. And they're going to leave on those terms'."