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Ben Chu
BBC Verify policy and analysis correspondent

As we reported earlier, the Liberal Democrats are today calling for the UK to join a customs union with the EU.

A customs union is where two territories sign an agreement to charge the same taxes on imports (tariffs) for goods from everywhere else in the world.

What are the advantages of a customs union?

Being in a customs union means the parties can then send on those goods without certain checks which is called “free circulation”.

The UK has a tariff-free post-Brexit trade deal with the EU. However, as it’s not in a customs union, goods are still checked at the border to ensure they are genuinely from the UK.

Joining a customs union with the EU would reduce the need for those checks meaning less red tape and costs for British businesses.

However, other post-Brexit non-tariff barrier checks such as safety and security declarations would remain.

And the disadvantages to the UK?

To some extent joining a customs union would limit the UK’s ability to run an independent trade policy – its import tariffs would need to match the EU’s.

It might have to revise post-Brexit free trade agreements with other nations if they face lower UK tariffs than those levied by the EU.

However, trade experts say there’s a difference between the UK joining “a” customs union with the European Union like Turkey has and rejoining “the” EU customs union.

The former would give the UK more freedom to trade but lower benefits. The latter would mean the EU negotiating future trade deals with other countries on the UK’s behalf.