What is a customs union and why does it matter?

In recent days, politics has been dominated by whether or not the UK would be in a customs union with the European Union after Brexit.

Theresa May has said she wants a “deep, special and bespoke” deal that looks a bit like a customs union for goods, while Jeremy Corbyn has called on MPs from all parties to support Labour plans for a new customs union with the EU post-Brexit.

Meanwhile European Council President Donald Tusk has warned there can be "no frictionless trade" with the EU if the UK is outside the customs union and single market.

On Friday, the Prime Minister will give a speech setting out her vision on the UK's future relationship with the EU in which she will likely give more information on whether she wants the UK to stay in a customs union or not.

So, what is a customs union and why does it matter? ITV News At Ten Presenter Tom Bradby explains.