LONDON — Boris Johnson took a swipe at the U.K. government’s Brexit plan today, referring to ongoing negotiations as the equivalent of wrapping “a suicide vest” around the British constitution and handing the detonator to Brussels.

Writing in the Mail on Sunday, the former British foreign secretary claimed Theresa May and her ministers had given in to the EU Brexit negotiator’s every demand, leaving the U.K. open to “perpetual political blackmail” in what amounted to a “humiliation.”

“We have wrapped a suicide vest around the British constitution — and handed the detonator to [EU Brexit negotiator] Michel Barnier,” Johnson wrote. “We have conspired in this threat to the Union. We have put our own heads deliberately on the block.”

May’s soft-Brexit plan would expose U.K. business to “potentially hostile regulation over which we have no control whatever,” according to Johnson. “It means we can’t do any real free trade deals,” he wrote. “It means we are a vassal state.”

The column is Johnson’s second intervention in a week, after writing in the Daily Telegraph on Monday that the Chequers plan would be a “disaster” for the U.K.

Johnson also criticized the so-called “backstop” option in case of a no-deal Brexit — a safety net to ensure cross-border cooperation between the Republic of Ireland and Northern Ireland — claiming that the EU is attempting to create a new border in the Irish Sea.

“There is no need for any kind of friction at the border at all,” Johnson wrote, calling on the government to turn to “technical solutions” relying on new technologies.

He also called on May to push for a “big Canada-style free trade deal.”

“Otherwise, we should tell our friends they won’t get a penny,” Johnson added.

In a separate column in the same newspaper, current Foreign Secretary Jeremy Hunt defended the government’s Chequers plan and called on the country to “unite behind May,” claiming she is “better than anyone I know at holding the line in the face of intense pressure.”