1) Staying in the customs union while leaving the EU would be the worst of all worlds. It would mean that Brussels continued to dictate our trade policy while we had no input into what that policy was. Plainly this is a hugely disadvantageous position, which is why no one else, even in countries that are in the single market such as Iceland and Switzerland, wants to join the customs union. At a time when the world's largest economies – the United States, China, India – are lining up to sign bilateral deals with the UK, surrendering our ability to do so would be crazy.
2) Actually, it's even worse than that. Staying in the customs union while being outside the EU means that we'd have one-sided deals with the states covered by the EU's existing trade deals, and be unable to improve them. Other than micro-states such as Monaco and the Vatican, the only non-EU state that is partly in the customs union is Turkey, which joined as part of a presumed journey to full membership. Turkey's customs union deal is weirdly lopsided. For example, when the EU signed a trade deal with South Korea, South Korean exports could enter Turkey on the same terms as they could enter the EU. But Turkish exports, unlike EU exports, are still subject to WTO tariffs when they enter South Korea, with no possibility of negotiating better terms.
3) If the whole purpose is, as Keir Starmer states, to "avoid a cliff-edge", then what is the point of maintaining the membership package in its entirety for years to come? The way of avoiding a cliff-edge is to have a phased and gradual repatriation of power. Keeping everything in place actually creates a precipice – it simply moves it further down the track.
4) No one could possibly have come up with this model from first principles, looking for the best deal for Britain. It is clear enough what is going on. Labour is holding out the prospect of a transition so long that it could be undone with no fuss, and the decision on Brexit reversed. After all its "country before party" talk, Labour is playing politics in an especially babyish way. It can't honestly believe that a customs-union-minus-any-voice deal is in our interest. All it wants is a win on something and a chance to divide the Tories.
5) The only effect of this kind of game-playing will be to embolden hardliners in Brussels and make an advantageous deal less likely. Just before the summer break, Donald Tusk, the President of the European Council, speculated about "whether Brexit could be reversed", going on to quote John Lennon's Imagine: "People say I'm a dreamer, but I'm not the only one." He certainly isn't. The EU is full of people who think that, if they offer a bad enough deal, Britain will drop the whole idea.
If you're reading this article in the UK, you'll know how preposterous that analysis is. There has been a referendum, public opinion hasn't shifted, and we are now in a legal process that will result automatically in withdrawal on 29 March 2019. But the EU media tell a very different story, of pro-EU demonstrations in London, of a scramble for foreign passports, of a supposed economic meltdown. Now here is the Official Opposition promising, in effect, to keep the door open. What impact is that likely to have on Eurocrats' negotiating tactics? Presumably it will make them toughen their stance in the hope that Britain might back down. But since there is no chance of Britain actually backing down, the only consequence will be that the eventual deal is worse for both sides.
To summarise, Labour is putting forward an idea that it knows to be unworkable, and whose chief consequence will be to make a beneficial deal less likely. Yet pro-EU columnists are, with dreary predictability, hailing it as mature and realistic and blah blah. No wonder the country stopped listening to them.
Daniel Hannan has been Conservative MEP for the South East of England since 1999, and is Secretary-General of the Alliance of European Conservatives and Reformists. Follow : @danieljhannan