Wild theories are doing the rounds about whether Britain should or should not join a permanent customs union with the EU after Brexit.

Unsurprisingly, Remain supporters have leapt at the idea as a way to soften Brexit. Some even hope that it would constitute such a bad deal that it might just lead to a reversal of Brexit.

Downing Street yesterday ruled out Britain either remaining in the customs union or joining a future customs union with the EU. Nonetheless, the spectre of the customs union continues to loom large. And there are a number of dangerous myths that are badly confusing matters.

Myth number one: Britain joining a customs union with the EU would solve the Irish border issue

The customs union issue refuses to go away because there still isn’t agreement on how to prevent a hard border on the island of Ireland after Brexit. But a customs union would not be sufficient to solve the issue.

If Britain were to remain in a customs union with the EU, there wouldn’t be any tariffs charged on goods travelling across the Irish border. But, given that Britain would have left the single market. there would be divergence in terms of regulation. As a result, goods would be subject to checks at the border. (That is one of the reasons why there are checks on the border with the EU and Turkey, which has a partial customs union with the EU.)

One could, of course, argue that the easy solution to all of this is for Britain to join the single market too. That would in effect deliver undisrupted trade, and many Remainers probably wouldn’t mind. But, as I explain below, that would be even more unsustainable than merely staying in the customs union.

Myth number two: Britain joining a customs union would at least keep the status quo for customs and trade

This simply isn’t the case. At the moment, as an EU member state, Britain can veto trade deals that the EU undertakes. That would no longer be possible if Britain was a in a common customs union with the EU. In such a scenario, Britain wouldn’t be able to block an EU initiative to, for example, start trade talks with Russia. Far from maintaining the status quo, such a scenario would effectively leave Britain powerless over its own trade policy.

Myth number three: Britain joining a customs union with the EU is a sustainable arrangement and would “soften” Brexit