In the recent Commons indicative votes on alternative Brexit options, the idea of remaining in the EU’s Customs Union emerged as the most popular future relationship with the EU even though, like every other option, it failed to get a majority. This is of course because it is Labour’s preferred option although Labour, fantastically, supports the idea only if the EU gives the UK a say in determining future EU trade policy (which it won’t). Labour supporters of the Customs Union rarely say anything much in detail about why they support this option beyond a vague intention to preserve jobs, though what these jobs might be is rarely made clear. In fact, there is little evidence that a customs union would be a good idea for the UK. The main arguments for a customs union are that it will guarantee tariff- and quota-free access for UK exports to EU markets and that it will avoid UK firms having to bear customs and ‘rules of origin’ costs that they would face in a free trade agreement (the latter involve the costs of ensuring product has enough ‘local’ content to qualify for zero tariffs). On top of this, it is claimed that a customs union solves the problem of the Irish border. In our view, the purely economic arguments in favour of UK customs union membership with the EU are weak: There is not much evidence that a customs union would be more beneficial for UK-EU trade than a standard free trade agreement (FTA). A large-scale academic study from 2006 finds no evidence that customs unions outperform FTAs, while a more recent study even suggests the EU customs union has a smaller trade creating effect than FTAs such as NAFTA (which covers North America). Rules of origin costs are often hugely overstated. Claims that rules of origin costs for UK businesses in case of a UK-EU FTA could be as high as 7-8% of trade values are far too high. A careful study by the WTO suggests such costs are less than 1% of trade values, and often negligible. Costs of customs processing are also massively exaggerated. Claims by HMRC last year that customs costs could total 1% of UK GDP or 6% of trade values are anything from five to twenty times too high; they are based on dubious calculations and are totally at odds with on-the-ground industry experience. A ‘new’ UK-EU customs union would not even remove customs-related costs. Formal customs checks within the EU customs union only ended in the early 1990s due to the Single Market Programme, and still exist in Turkey’s customs union with the EU. Moreover, the documentary requirements associated with trading in a customs union can actually be greater than for trading on WTO rules! The UK’s foreign trade structure is not suited to a customs union. Customs union arrangements have some logic where one economy does a very large share of its trade with another. But the EU now represents only around 45% of UK goods exports and this share has been dropping rapidly. Twenty years from now it is likely that the EU will take only around a third of UK goods exports. The UK would remain locked into the EU’s highly protectionist agricultural trade system. High EU tariffs on agricultural products represent a heavy ‘tax’ on UK consumers. UK consumers are denied the choice of cheap food from outside the EU and pushed towards consuming expensive products from within it. This cost is high at 0.5-1% of GDP. Moreover, the strategic/political arguments in favour of a customs union are even less compelling: Entering a customs union would make meaningful trade deals with other economies impossible. There could be deals on trade facilitation or deals on services but their scope would be very limited. Why would India or the US be interested in a deal on services (potentially benefitting the UK) when the UK had nothing to offer on the goods side? The EU would be able to ‘sell’ access to UK markets with no reciprocal benefits for the UK. Britain would be in the same boat as Turkey: when the EU does trade deals with third parties, these countries gain tariff-free access to Turkish markets but Turkish exporters do not gain automatic reciprocal access to these third countries. Britain would have no voice at future WTO discussions about global tariffs. It would simply have to accept whatever the EU agreed. The EU would be able to damage UK business using anti-dumping actions. Under a new UK-EU customs union the EU would be in charge of the UK’s ‘trade defence’ measures such as ‘anti-dumping’ actions. The EU could force the UK to impose steep tariffs on goods from third countries, hurting UK businesses and consumers. Worse still, the EU might insist on being able to impose anti-dumping duties on the UK as well – as is the case with Turkey. A customs union would not simply cover tariffs and quotas. The EU would also require the UK to follow EU rules in a broad swathe of policy areas including competition policy, environmental policy and social and labour standards – without any say. This would not only be a huge loss of UK sovereignty but also dramatically narrow the UK government’s freedom of action in key economic policy areas. A customs union does not solve the Irish border ‘problem’. Customs checks only represent a small element of potential border checks at EU borders today. A bigger issue is product conformity and other single market rules. This is another reason why any customs union would require either effective UK single market membership or border checks between Britain and Northern Ireland and/or Britain and the rest of the EU. In summary, a customs union arrangement whereby the UK contracted out huge areas of trade and economic policy-making to the EU would be totally unsuitable for an economy like Britain’s. Customs union arrangements may work well for small economies that do an overwhelming share of their trade with a large neighbour. But the UK is the world’s fifth largest economy, with a diverse pattern of foreign trade and with business and consumer interests that will often diverge from those of the EU. It is no accident that Canada and Mexico are not interested in joining a customs union with the US, despite their strong trade orientation towards the US. They know that the loss of economic independence involved would be far too great to justify a modest reduction in border frictions. The calculation should be the same for the UK. Supporters of a customs union have suggested the UK could somehow retain some influence over decision making in such a new UK-EU arrangement. But this looks like a fantasy. It would be legally and politically difficult for the EU to grant any significant decision-making power to the UK. The best the UK could hope for would be some kind of observer status. But the arrangement would remain a thoroughly one-sided one where the UK would have no power either to veto potentially damaging agreements or to push for deals that benefited it. Entering a new customs union with the EU would be a backward-looking step for the UK, with a massive loss of policy independence and flexibility while leaving businesses and consumers at risk of having damaging decisions imposed on them with no say in how those decisions were taken. It would also give the UK minimal additional policy freedom in the trade and economic policy area. Overall, it is hard to imagine a more sub-optimal policy.