“Brexit will dramatically alter the UK’s trading relationship with its biggest trading partner”, the EU, according to a new report from the EU Committee of Britain’s House of Lords.

The Committee is publishing a series of reports on aspects of Brexit – starting with its impact n Ireland (published yesterday).

The report warns that Britain will not be able to cut itself off from all forms of regulation and the jurisdiction of international trade bodies, no matter what form of Brexit it adopts.

It says the priority for the British government is to sort out its future trading relationship with the EU – its biggest trading partner by far.

Because of the complexity of this, it recommends trying to secure an early transitional agreement that will give the UK and its trade partners more time to deal with trade arrangements than the two years allowed by Article 50 – the two-year process under which Britain will negotiate its exit terms from the EU.

The House of Lords Committee also zeroes in on the EU Customs Union as a critical – perhaps the critical issue in the EU-UK trade relationship post Brexit.

It urges the UK government to make an early decision on whether or not the UK should seek to remain part of the Customs Union.

This is also a key issue for Ireland as it would have huge implications for the agri-food sector, which would face very high tariffs on UK-Irish trade.

The report states: “We are concerned that the Government appears not yet to have given sufficient consideration to the implications of leaving the EU’s customs union.

“While there may be opportunities to use digital technologies to streamline customs procedures, we are troubled that the Government presently has no estimate of the cost to businesses of administrative delays, compliance with customs checks, and the rules of origin if the UK left the customs union, and that it was unable to confirm whether or not such information would be available before triggering Article 50.

“Our concerns are made more acute by the implications of leaving the customs union for the UK’s land border with the Republic of Ireland.”

It also urges the British government to carry out and report a rigorous analysis of the cost to businesses and taxpayers of leaving the Customs Union – before it triggers Article 50.

Read Sean Whelan's full assessment of the House of Lords Committee report on post-Brexit trade here.