Among any group of Remain politicians, discussion soon turns to the likelihood of stopping Brexit. After Theresa May's electoral humbling, and the troubled start to the negotiations, those who oppose EU withdrawal are increasingly optimistic.

“I’m beginning to think that Brexit may never happen,” Vince Cable, the new Liberal Democrat leader, said recently. A growing number, including those who refuse to comment publicly, are of the same view.

But conversation rarely progresses to the potential consequences of halting Brexit. The assumption that the UK could simply retain the status quo is an unsafe one. Much hinges on whether Article 50 is unilaterally revocable (a matter Britain might have been wise to resolve before triggering withdrawal.) Should the UK require the approval of the EU27 to halt Brexit (as some lawyers believe), or be forced to reapply for membership, Brussels would extract a price.

Guy Verhofstadt, the European parliament’s Brexit co-ordinator, recently echoed French president Emmanuel Macron's declaration that “there is always a chance to reopen the door”. But he added: “Like Alice in Wonderland, not all the doors are the same. It will be a brand new door, with a new Europe, a Europe without rebates, without complexity, with real powers and with unity.”

The UK's £5bn budget rebate, achieved by Margaret Thatcher in 1984, has long been in the EU's sights. A demand to halt Brexit would provide the perfect pretext for its removal.

As Verhofstadt's reference to “unity” implied, the UK's current opt-outs would also be threatened. At present, Britain (like Denmark) enjoys the right to retain its own currency and (like Ireland) an exemption from the passport-free Schengen travel zone. Were the UK to reapply for membership under Article 49 of the Lisbon Treaty, it would be automatically required to join the euro and to open its borders.

During last year's Labour leadership election, Owen Smith was candid enough to admit as much. “Potentially,” he replied when asked whether he would accept membership of the euro and the Schengen zone as the price of continued EU membership (a stance that would not have served Labour well in the general election.)

But despite the daily discussion of thwarting Brexit, politicians are rarely confronted by such trade-offs. Remaining within or rejoining the EU, like leaving, is not a cost-free option (though it may be the best available.) Until anti-Brexiteers acknowledge as much, they are vulnerable to the very charge they level at their opponents: that they inhabit a fantasy world.