Members of Theresa May’s cabinet apparently have “significant reservations” about her Brexit transition plan. That is hardly news: all policies involve reservations. But to describe a customs union fall-back as vassalage, bondage and enslavement insults both language and history. May’s colleagues are at this critical moment seeking to undermine her attempt to find a way through a political nightmare, and with no proffered alternative. That is anarchy.

Irrespective of whether a customs union with Europe is good for Britain – and evidence and common sense suggests it is – an interim fall-back to no customs union is utter stupidity. It would mean border chaos, massive disruption to food supplies, damage to exports and yawning gaps in labour supply. A mix of personal ambition and reckless brinkmanship may appeal to hot-blooded ministers. But that is not responsible government.

Negotiating Brexit was never going to be easy. The case for a political breach with the EU was emotional rather than rational, but at least it could work. The UK simply ceases to be a member of the EU next March. But two years on from the referendum, the case for an economic breach remains elusive. There are no other market openings that could possibly replicate the EU’s now complex single market. May’s own pretence that such a “unicorn” exists was always fantasy. Now her unicorn has come home to graze.

Even so, she and the country are entitled to expect those selected to lead the government to keep the public interest at heart. The border with Ireland was always a clincher, because Northern Ireland exposed the reality, that Britain cannot both be part of the EU economy and not part of it. It must be Cyprus or not Cyprus. No Brexit plan has resolved this satisfactorily for the simple reason that it is not resolvable. Two further years of transition negotiations will probably not do so either. If we wish to trade friction-free with the EU, we must do so on its terms. Trade is a barter, not a shackle. The issue is balance of interest, and that must lie in a customs union.

Party discipline is now disintegrating. The Tories’ favourite “secret weapon”, loyalty, has gone blunt. Sooner or later, every MP will have to stand up and be counted, “deal or no deal”. The latter would not be the end of the world, but it would plainly be disruptive and very expensive – and yield no conceivable public benefit. It is astonishing that those elected to run the country, including as paid servants of the crown, can seriously contemplate it.

• Simon Jenkins is a Guardian columnist