BRUSSELS (AP) — The great Brexit brawl is heading into its next standoff on Wednesday when European Union concessions to improve trade in Northern Ireland look likely to be deemed insufficient by the United Kingdom.

Officials said that the EU's top Brexit official Maros Sefcovic is to propose major practical changes in the Byzantine system of customs and checks in Northern Ireland, which is U.K. territory but remained part of the EU's borderless trading market when the U.K. left the bloc last year. Under the new rules, goods must be checked between Britain and Northern Ireland.

However welcome such a cut in red tape might be to London, it is also insisting on a further, fundamental change. It wants the EU to cede final legal oversight of any trade disputes there, and leave it to independent arbitration.

EU officials, member nations and the bloc’s parliament have bristled at the thought that the EU's Court of Justice would lose its preeminence over part of its trading market, and Sefcovic's room for maneuver is narrow in negotiations starting later this week.

"The role of the ECJ ... is not up for negotiation”, said Nathalie Loiseau, a key EU parliamentarian from France.

Both Britain and the EU agreed it was essential to avoid a hard land border between Northern Ireland and EU member Republic of Ireland, since the island long was the scene of deadly sectarian violence before the border posts were brought down as a key part of the 1998 Good Friday agreement.

The EU thought years of Brexit wrangling were over when the government of British Prime Minister Boris Johnson officially signed the legally binding divorce agreement and the subsequent free trade deal at the end of last year.

Now, the EU says Johnson is seeking to row back on the Northern Ireland deal that he himself signed.

Charges of British duplicitousness and were given even more fodder on Wednesday when Johnson’s former top aide Dominic Cummings suggested the U.K. government never intended to honor the withdrawal agreement it signed up to.

"That would indicate that this is a government, an administration, that acted in bad faith and that message needs to be heard around the world,” Irish Deputy Prime Minister Leo Varadkar told Irish broadcaster RTE.

"This is a British government that doesn’t necessarily keep its word and doesn’t necessarily honor the agreements it makes,” he added.

France has similarly said that Britain can't be trusted in a dispute over fishing licenses that Paris said should have long been granted as part of the signed trade deal with London. French spokesman Gabriel Attal said the government intends to announce retaliation measures before the end of the month.

It is in such a heated context that Sefcovic now must negotiate with his U.K. counterpart David Frost to smooth trade in Northern Ireland.

He will be traveling with wide-ranging proposals to cut the paperwork for goods entering Northern Ireland, and eliminate impediments that have created irritants — such as when British Cumberland sausages couldn't make it onto Belfast dinner tables because of EU rules on chilled meat.

Jill Lawless in London, and Sylvie Corbet in Paris, contributed to this report.

Follow AP’s coverage of post-Brexit developments at https://apnews.com/hub/Brexit