Image copyright Niall Carson Image caption Theresa May has reiterated the government's determination to avoid any return to "the borders of the past"

A former head of the European Commission's customs procedures unit has claimed that controls at the Irish border will return post-Brexit.

Theresa May has said Brexit means leaving the European Customs Union.

It currently allows tariff and paperwork-free trade between the UK and the Republic of Ireland.

Michael Lux said "red and green lanes" could be introduced as a form of customs control.

"So there would be two lanes for vehicles; a green one where you are only occasionally stopped and a red lane if you carry goods above the threshold," he told the BBC's Inside Business programme.

Image copyright Niall Carson Image caption A former customs post lies derelict in Jonesborough, Co Armagh, on the northern side of the border

"Such a system can be established between Northern Ireland and the Republic of Ireland."

Theresa May has reiterated the government's determination to avoid any return to "the borders of the past".

On Friday, Taoiseach Enda Kenny told RTÉ that "papers are being prepared" on ways to minimise border customs checks.

What is a customs union and why does it matter?

A customs union is a form of trade agreement between two or more countries.

It means they decide not to impose tariffs (taxes on imports) on each other's goods and agree to impose common external tariffs on goods from countries outside their customs union.

Setting common external tariffs is what distinguishes a customs union from a free trade area.

The key argument for leaving the customs union is that it will allow the UK to negotiate its own trade agreements.

Michael Lux suggested the Norwegian-Swedish border could be recreated on the island of Ireland.

"In principle, there will be Irish and UK customs officials on either side of the border," he said.

Image caption Svinesund bridge on the Oslo to Gothenberg motorway

"But they could agree to create only one stop and officials from both sides deal with the matter or one country allows the other to act on their behalf."

Last October, former first minister Arlene Foster said she didn't foresee a time when heavy goods vehicles would be stopped and searched post-Brexit.

But Mr Lux said it was unrealistic to expect no delays at the border with the Republic of Ireland.

"Yes it can be done electronically with an export declaration, but from time to time, there must be controls."

Inside Business will be broadcast on BBC Radio Ulster at 13:30 GMT on Sunday.