The question, firstly, is whether one means "the" EU Customs Union, or "a" customs union with the European Union, since the motion before the House of Commons on Monday from Conservative MP Kenneth Clarke was for the former, not the latter.
Currently, the UK can trade throughout the EU, while applying the same level of tariffs for countries outside the EU as the rest of the union. Internally, it means that there are no customs delays, no checks.
Such freedoms are particularly significant for businesses such as the car industry, which are dependent on parts arriving at plants throughout the midlands and elsewhere minutes before they are needed on the factory floor.
Acceptance after Brexit of the EU’s common external tariff would mean that the UK could not lower tariffs on imports from non-EU member states – something which Brexiteers have argued is one of the fundamental reasons why the UK should get out of the EU in the first place.
Lack of detail
If “the” EU Customs Union continues to remain unpalatable, some in the House of Commons argue for “a” customs union, though the lack of detail on offer about what they mean is striking by its virtual absence during the debate.
Turkey could be an example. It has a partial customs union with the EU, charging EU-approved tariffs on most industrial goods coming from countries other than those in the EU – unprocessed agricultural products are not included.
Turkey does enjoy advantages from the deal, sharply reducing the friction on trade that it does with the EU, one of its biggest markets. In return, however, it has had to concede that the European Union controls many of its trade policies to a significant degree. The country, for example, is forced to open its market to third countries which have signed free-trade deals with the EU. The EU enjoys benefits from such deals, since trade by its companies is facilitated. Turkey, however, is not guaranteed such reciprocal benefits.
