Theresa May is on the ropes over Brexit. From her election disaster last June to the ridiculed reshuffle this week, the prime minister must feel as if Brexit is a curse which nothing will lift.

On the old political maxim that the best time to kick an opponent is when he or she is flat on their back, one might expect Jeremy Corbyn would relish putting the boot into the woman he would love to replace. But instead he insists on offering her a helping hand by making a statement on Brexit that confuses and dismays most of his MPs while leaving the government without any opposition on the most important issue of the day.

On the day of the botched reshuffle Corbyn decided to make his own headlines by telling a meeting of Labour MPs that Britain cannot stay in the single market under any circumstances and that leaving the EU means leaving the single market. This remarkable assertion – common enough on the lips of Nigel Farage or Michael Gove – is legally and politically nonsense on stilts.

Norway, to take one small example, is not in the EU but is in the single market. It is perfectly possible for the UK to leave the EU politically but stay within the single market.

Leaving the EU as a treaty organisation means not participating in the European Parliament elections, not sending ministers to the EU Council of Ministers and not nominating a European Commissioner. That meets the 2016 referendum mandate to “leave” the EU.

Nowhere on the ballot paper were the words “single market”. Indeed soon after the referendum, Boris Johnson, wrote in the Daily Telegraph: “There will continue to be free trade, and access to the single market.” His fellow Tory Brexit cheerleader, Daniel Hannan MEP,  declared: “Absolutely nobody is talking about threatening our place in the single market.”

The defence of Corbyn’s endorsement of leaving the single market is that he cannot go against the votes of the 37% of the total electorate who opted for Brexit. Yet by repudiating staying in the single market he is going against 65% of Labour voters who voted Remain. In a YouGov poll published late in December, two-thirds of those who voted remain said they would be disappointed or angry if Labour says it will proceed with Brexit.

One cannot help thinking that a more commanding, cannier Labour leader like a Harold Wilson or a John Smith would be able to excoriate the Tory government on the threat to investment, jobs, wages and social rights that a fully fledged Brexit entails, while simultaneously keeping alive staying in the single market or even consulting the people once we know what Brexit is, without explicitly repudiating the June 2016 vote.

Corbyn appears not to have these skills. Last year was his annus mirabilis. Will 2018 be the year when “Oh, Jeremy Corbyn”  becomes “Oh dear, Jeremy Corbyn”?

Denis MacShane is a former Minister of Europe and was a Labour MP for 18 years.