When a country joins a customs union (CU), the acceding member adopts the CU tariff schedule. Where this results in a loss of market access for third countries, because custom union tariffs are higher than the bound tariffs the acceding country had scheduled in the WTO, third countries have a right to seek compensation (for example, countries such as Australia, Argentina, Brazil, China and Uruguay submitted claims for compensation when Croatia acceded to the EU in July 2013). All contingency trade measures (antidumping, anti-subsidy and safeguards) equally apply to the acceding members.

In the case of quantitative market access commitments, such as tariff rate quotas (TRQs), these are conventionally added to those of the CU. Similarly, commitments in the areas of domestic support and export subsidies are added to those of the CU.

Going in the opposite direction following the exit of a CU member is not so easy, particularly when that member was a member of the EU when the current WTO commitments were agreed following the Uruguay Round in 1994. There is no evident baseline to which these commitments can be rolled back. So how to establish what the UK’s agricultural policy WTO commitments would be following a possible Brexit?