Suzanne Breen: Customs union solution wouldn't resolve the hard border problem
In the Tories' 2017 general election manifesto, Theresa May pledged that the UK would no longer be a member of the customs union after Brexit.
But in an attempt to break the parliamentary paralysis over Brexit the Prime Minister is holding intensive talks with Labour leader Jeremy Corbyn in an attempt to agree a joint way forward.
The UK remaining in a permanent customs union with the EU is a central core of Labour policy. So any agreement between Mrs May and Mr Corbyn is likely to involve the Prime Minister erasing, or at the least blurring, one of her red lines.
Attorney General Geoffrey Cox has said that a customs union isn't desirable, but that if it's the only way of leaving the EU he would accept it.
So what is a customs union? Its purpose is to make trade easier. Countries in a customs union agree not to impose customs checks or charges - taxes on imports and exports known as tariffs - on each other's goods.
The EU's customs union includes all member states, plus Monaco. Goods within these countries can travel around unimpeded.
The rules dictate that any goods coming in from elsewhere in the world pay the same tariff. It doesn't matter in which country in the customs union the produce first enters. This is called the common tariff.
The EU insists that members of the customs union can only negotiate as a whole with the rest of the world, and cannot choose to negotiate tariffs with outsiders.
Eurosceptics say the benefits of leaving the customs union would be considerable, giving the UK the freedom to sign lucrative, tailor-made deals with the US and China, which boasts a booming economy.
The Prime Minister had previously argued that remaining in the customs union would hinder the UK's ability to strike deals. It is the logjam in Parliament that has brought her to discussions in which it's on the table.
So could the UK strike trade deals while in the customs union? Because the UK would not be able to negotiate tariffs, it could only make deals on other areas, such as regulations.
Stefan Enchelmaier, a professor of European law at Oxford University, agreed that this leaves Britain with little bargaining power.
But he added: "The UK would be free to allow in chlorinated chicken and hormone beef."
These are two infamous examples of goods that could be imported from the US if the UK relaxes regulations post-Brexit. They are currently banned in the EU.
Prof Enchelmaier also pointed to other difficulties in negotiating trade deals, saying the EU has faced difficulties with India because India wanted certain levels of freedom of movement along with trade.
"These concerns are not going to go away when the UK faces India," he said.
But even if the Prime Minister and Mr Corbyn reach an agreement on the customs union it does not solve the problem of a hard border on the island of Ireland.
That issue would remain unaddressed because checks would still need to be carried out on safety standards when goods crossed the border.