It was the big Brexit lie. No, not the £350m a week to spend on the NHS or the “bonfire” of red tape. The lie was that the shambles now enveloping British trade with Europe was an unavoidable price worth paying to leave the EU. That was rubbish. In order to further his chances of becoming Tory leader Boris Johnson made two commitments. One was to resign from the EU, the other was to depart Europe’s customs union and single market, aspects of which embrace other non-EU states such as Norway. The second decision was an almost casual gesture to make him look macho to the party’s hardline Brexiters. It was not put to referendum and was beyond stupid. No news item today is free of the consequences. Earlier this year, the effects of leaving the single market could be seen in plummeting trade with the continent, even accounting for the pandemic. Additional red tape is awesome. HMRC estimates traders will be handling 215m more import/export documents a year, at an estimated bureaucratic cost of £7.5bn a year. Tariffs may not apply but rules of origin and health standards do. Every truck, every cargo requires inspection. As for migration, the overall shortage of seasonal farm labour, according to BBC Radio 4’s Farming Today, is 20% and often more. Fruit will rot in fields, pigs cannot get to abattoirs and Christmas turkeys will be a “nightmare”. Meanwhile, care homes in England are short of 170,000 staff, and delivery firms short of 100,000 drivers. Hotels have abandoned rooms and restaurant tables. Creative industries – worth £110bn to the UK economy – were forgotten by the Brexit negotiators and are now virtually isolated from Europe. This is not Brexit. Britain could have left Brussels and freed itself from a mass of rules and regulations. It is the result of leaving the single market, of Johnson’s xenophobic belief that European trade standards were somehow “not British”. He was wildly in favour of EU workers when mayor of London but no longer as prime minister. I am sure some of the current disruption will settle down but the idea that trade with Britain’s biggest partner by far, the EU, will ever recover outside some form of economic union is absurd. So is the theory that any losses from the present chaos will be met by gains elsewhere. It seems bizarre to have to explain to a Tory that prosperity lies in open markets not closed ones. Johnson has significantly not set up a permanent trade and agriculture commission to guard British interests in new deals. He is clearly desperate for deals, however bad. In addition, the National Audit Office has yet to do the normal impact assessment of Lord Frost’s post-Brexit deal with the EU. It has not bothered. I imagine the assessment would be the colour of blood. Brexit need never have so devastated the British economy. The damage has come from one decision, to depart the single market. The sensible path now would be for Johnson to eat humble pie and seek, as far and as fast as possible, readmission to that market. Britain would imitate the protocol it has agreed for Northern Ireland. This would not mean rejoining the EU, just rejoining Ireland – the most delicious of historical ironies. Negotiating the single market in 1987 was Margaret Thatcher’s proudest free-trade achievement. It was in Britain’s and Europe’s interest and proved a success. Johnson reversed that achievement in an act of naked political ambition. He pretended it was necessary for Brexit. It was his biggest lie. Simon Jenkins is a Guardian columnist