Government admits it is checking if Theresa May's Brexit customs plan is legal

David Lidington, the prime minister's 'deputy', says it will be a 'week or so' before proposals are fully examined - after ministers made 'serious criticisms'

Lawyers are examining whether Theresa May’s plan for trading with the EU after Brexit is legal under international law, her deputy has admitted.

David Lidington, the cabinet office minister, also acknowledged that senior ministers had made “serious criticisms about the technical detail” behind the so-called “customs partnership”.

It would be a “week or so” before the inner cabinet was ready to fully examine both the partnership plan and the rival “max fac” model based on smart technology, he said.

However, Mr Lidington denied the involvement of lawyers presented a further hurdle, saying: “We test the legal risks involved. This is just a normal routine part of policy

“This is what always happens when you talk about a treaty. You check is this compatible with other treaties to which you are party, or the countries you are negotiating with are party.

“This is not something that is special here - this is a normal part of everyday government business.”

It is believed that David Davis, the Brexit Secretary, has told Theresa May that the customs partnership – under which the UK would collect tariffs for the EU - could be illegal.

A letter, setting out his opposition to the proposal, warns that, if the government backs the partnership plan now, it would be too late to reverse the policy if legal obstacles then emerged, it has been reported.

The attorney-general will also consider the legality of the maximum facilitation model, to establish if it could also be challenged at the World Trade Organisation (WTO).

Speaking on BBC Radio 4’s Today programme, Mr Lidington also ruled out seeking to extend the post-Brexit transition period beyond the end of 2020, if the customs impasse continues.

He spoke after Mr Davis announced a white paper on Brexit strategy would be published next month, to rebut allegations that Britain has failed to state what kind of future relationship it wants.

But there is no guarantee the document will settle the row over future customs, as two small ministerial working groups continue to examine the flaws in both models.

Meanwhile, Labour’s shadow Brexit minister cast further doubt on the party’s stance on the looming Commons vote on whether to stay in the EU single market.

This week, Jeremy Corbyn’s spokesman insisted he would not bow to pressure to back membership – but Paul Blomfield said Labour was “ruling nothing off the table”.

Mr Blomfield repeatedly sidestepped questions over whether Labour would back making membership of the European Economic Area (EEA) - and therefore the single market - a priority, when the issue comes before MPs.

“We are going to make our decision on the amendments that come back from the Lords when the Government brings the legislation back from the Lords. We are ruling nothing off the table,” he said.