The Windsor Framework did not correct an “underlying error” in the Northern Ireland Protocol, a Westminster committee heard.
The House of Lords European Affairs Subcommittee on the Windsor Framework heard the claim during a public evidence session as part of its inquiry into regulatory drift.
Criticism has also been heard about the amount of information given to businesses trying to work with post-Brexit trade arrangements.
Unionists in Northern Ireland strongly criticized the protocol, with the DUP walking out of the Stormont Assembly in protest.
Prime Minister Rishi Sunak earlier this year resumed negotiations with the EU and agreed to the Windsor framework.
But DUP leader Sir Jeffrey Donaldson is maintaining his party's protest and is calling for further action from the UK government on trade unionists' concerns over trade deals and Northern Ireland's separation from the rest of the UK.
James Webber, a partner at law firm Shearman and Sterling, said he was “pessimistic” about what the Windsor Framework had achieved.
He said he believed the framework “embodied a fundamental analytical error that was made when the Northern Ireland Protocol was first designed”.
“The practical implications of this mistake will, I think, get worse over time,” he told his peers, describing Northern Ireland as a “no man's land” that does not exactly copy either Great Britain or the EU.
“That fundamental analytical error I am referring to is to accept as the Windsor Framework does, and as the protocol before it did, the application of foreign law and foreign courts to parts of the UK.”
He continued: “Much of the work of the Windsor Protocol is to ease east/west trade flows.
“None of this really changes the fact that EU law, essentially a foreign system of law, applies to the production of goods within Northern Ireland, and I think the dead-end cost of trying to work out which rules apply to you and in which direction divergence affects your business is likely to discourage investment and reduce trading volume and favor those incumbents who are finally able to thread the needle and work out how the system works relative to new entrants, and that over time it will reduce competitiveness, productivity, economic and wage growth.
“So I'm pessimistic, I'm afraid that in relation to what the Windsor Framework has achieved, it doesn't mean it hasn't achieved anything, but I think it reflects an underlying error, which remains unfixable.”
At the same session, Anton Spisak, head of political leadership at the Tony Blair Institute for Global Change, said regulatory drift was “now a fact of life simply because the UK has left the European single market and the customs union, and is under no obligation to follow EU legislative changes”.
In an earlier session, Joel Reland, research fellow at UK in a Changing Europe, was asked about the ability of a future Stormont executive to align when there are rule changes in either the UK or the EU.
“There's currently no strain, which means we're not really in a position to assess it,” he said.
“So when the executive, at some point probably comes back to power, they will have to learn how to make policy if they want to maintain alignment with Great Britain and that's really, really, really hard to do.”
Committee chairman Lord Jay of Ewelme agreed with this point, adding: “I think your last point is something that has bothered the committee a lot in recent months.”
Meanwhile, there has also been criticism of the government over the amount of information on businesses.
Mr Reland said there were some “significant deviations”, such as the new state aid and procurement regimes where the government had partnered with business.
“But when in more day-to-day policy-making, things happen and there's a potential element of divergence, I'm not sure business has been consulted as closely as it should be,” he said.
Dr Lisa Claire Whitten, a post-Brexit governance researcher at Queen's University Belfast, said the process of dealing with regulatory diversions was “reactive rather than strategic”.
“To date, it's been piecemeal and ad hoc in terms of how it's been addressed,” he said.
“From a Northern Ireland perspective, there isn't even an official comprehensive record of what law applies under the protocol and framework, because that's not a service that legal.gov.uk currently provides.”
He added: “But we are at a point now where we have relative legal certainty about the arrangements and about the wider UK/EU relationship, so now is the time to develop that strategy and deal with this issue so it doesn't get worse, and there is a possibility of that.”