Brussels seeks permanent post-Brexit customs union

Sam Lowe, a senior research fellow at the Centre for European Reform think tank, said the constraints of an UK-EU customs union were substantial. “Whilst in a customs union with the EU, the UK will be required to fully align its external tariff with that of the EU’s, meaning it cannot lower or remove tariffs either unilaterally or as part of a new free trade agreement.”

The UK could negotiate trade agreements with countries like the US and China in those areas “not constrained by its customs union with the EU” — areas such as services, procurement, intellectual property, data and investment.

But even in a customs union with the EU the UK would not automatically get access to all the countries the EU has trade agreements with.

“The UK will need to replace all of the existing EU free trade agreements, and will need to mirror all future EU free trade agreements, while all those countries would continue to get, theoretically, continued preferential access to the UK market,” Lowe said.