Members of the customs union impose what's known as a common external tariff.

It's an ironclad rule of customs unions, which are essentially just free trade zones with common external tariff regimes.

Because without a common external tariff, internal border controls would be necessary to avoid what is known as trade deflection.

In a customs union, member states agree to a common external tariff, enforced along their common border with the rest of the world.

All that has to be done at the EU level, because that is the only way to maintain the consistency of the common external tariff.

It has launched a four-week consultation on the UK Global Tariff, which will replace the EU's Common External Tariff.

She said the country could no longer be bound by the customs union's Common Commercial Policy or its Common External Tariff.

The customs union includes non-EU Turkey, Andorra and San Marino who trade freely with the bloc and share its common external tariffs.