How Boris has escalated the cabinet war with a very public attack on May’s customs plans

The Foreign Secretary made clear that pushing ahead with the partnership model would betray his vision for Brexit.

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He’s mad as hell and he’s not going to take it anymore! Fresh from a set-piece interview in which he vainly attempted to rescue one messy multilateral deal dismissed by some as unworkable, Boris Johnson has moved to bury another.

Unfortunately for Theresa May, it was the Iran nuclear deal that her Foreign Secretary tried to salvage on Fox News yesterday, and her Brexit customs partnership at which he launched a cruise missile.

In an interview with the Mail – who else? – in Washington, Johnson escalates the cabinet war rumbled on over the weekend. He brands the Prime Minister's proposed model, which would see the UK collect EU tariffs at the border on behalf of Brussels, “a crazy system” and warns that it would render Liam Fox's existence pointless.

“It's totally untried and and would make it very, very difficult to do trade deals,” he says. “If the EU punitive tariffs on something the UK wants to bring in cheaply, there's nothing you can do. That's not taking back control of your trade policy, it's not taking back control of your laws, it's not taking back control of your borders and it's actually not taking back control of your money either, because tariffs would get paid centrally to Brussels.”

The subtext, if it's not overstatement to call it such, is clear. Though there is no explicit threat to resign, Johnson has made clear that pushing ahead with the partnership model – already rejected by May's war cabinet last week – would betray the vision for Brexit of which he is the self-anointed guardian.

But although he is pushing collective responsibility to its limit by attacking the Prime Minister so publicly – the first time any member of the inner cabinet has done so – the intervention means much more for May than it does for him. It is the clearest sign yet that can-kicking will no longer cut it.

Something will have to give if she is to present a workable customs deal to next month's EU council, and, given the tone of Johnson's intervention (which will delight Jacob Rees-Mogg and the ERG), it is unlikely to be the resolve of the Brexiters in the inner cabinet. It bodes terribly for Olly Robbins' last-ditch attempts to win them over before its meeting a week tomorrow, which May has deferred from this week in the hope of finding agreement in the interim.

It isn't just on her Leave flank that May has run out of road. It's worth remembering that Johnson is on the same page as the EU27 on the infeasibility of the customs partnership.

Meanwhile, the EU withdrawal bill reaches its final day in report stage in the Lords today, with three cross-party amendments demanding Britain sign up to continued membership of the EEA. Increasing numbers of Labour peers are inclined to rebel against any whip to vote against them and, should they pass, pro-EU rebels are confident they have the numbers on the Tory benches to win in the Commons.

Boxed in both sides, the time has come for May to find a new compromise – or accept that brokering a deal with the EU will mean losing friends and alienating people on her own benches.

Patrick Maguire is the New Statesman's political correspondent.