Leaving Northern Ireland with “one foot” in both the EU and UK markets under the Brexit deal will be “incredibly complicated”, the House of Lords EU committee has heard.

The UK committee heard from experts on trade and law that under the Irish Sea trade border arrangements in the deal’s Irish “protocol” some 300 pieces of EU legislation and EU customs regulations amounting to some 2,000 pages would need to continue to apply in Northern Ireland.

“There are still aspects of all drafts of the protocol that I can honestly say I do not fully understand how they are going to work in practice and what their implications are,” Sylvia de Mars, a law lecturer at Newcastle University, told the committee of British peers at Westminster. “It is a tremendously complicated piece of legislative work.”

The reworked arrangements were agreed in October to avoid a hard land border on the island of Ireland by keeping the North legally in the UK market but in practice in the EU market. The new arrangements are due to come into effect at the end of December 2020.

David Henig, a director of the European Centre for International Political Economy think tank, told the committee the implications of the “heavy inter-relationship” between the Northern Ireland protocol and EU law “is not always made obvious” and have “not been well studied”.

He told the committee that implementing the Brexit agreement was hampered by a “real lack of a very detailed guide as to what all this means”.

Mr Henig said “you just realise how complex” the arrangements for Northern Ireland are when you see that the protocol “interfaces” with the 1998 Belfast Agreement, the Common Travel Area with Ireland, World Trade Organisation rules and, in future, with a EU-UK trade deal.

Several months He said in places in the protocol there was a simple reference “innocuously” referring to an article of EU law applying in Northern Ireland but that this actually referred to “the entire customs union and laws of the European Union”.

“Understanding this would be a matter of several people taking several months,” he said.

The text was “not at all clear” on how a business with a headquarters in London and a subsidiary in Belfast would operate under the arrangements. “Everything is going to be very complicated.”

Colin Murray, another law lecturer at Newcastle University, said the arrangements put Northern Ireland “at the centre of a very complicated and convoluted Venn diagram” where the EU and UK market rules would “overlap in the years to come” without clarity on how things would work.