Traders selling goods between Northern Ireland and the rest of the UK will face "real hardships" in the coming weeks after the post-Brexit regulatory border shattered normal operations, lobby groups said on Wednesday.
At a parliamentary committee, representatives for the province's retailers, farmers and logistics firms said British companies had not been able to prepare, or were not prepared, for new customs requirements after a last-minute trade deal.
Trucks have been sent back to Britain, some have been held for hours while they fill out forms and other suppliers have stopped servicing Northern Ireland until the new systems bed in.
"This is just the opening skirmishes," said Aodhán Connolly, director of the Northern Ireland Retail Consortium.
"Retailers have been stocking up before Christmas for this first week, the flow of the first weekend was less than 20 per cent of usual transport flow, so there are real hardships that are going to come in the middle of this month."
Britain left the European Union's single market and customs union at 2300 GMT on New Year's Eve, introducing a raft of paperwork and customs declarations for those businesses that import and export goods with the bloc.
In order to keep the border open between Northern Ireland and EU-member Ireland, a separate agreement was struck that requires a regulatory border in the Irish Sea between Northern Ireland and the rest of the UK.
While stockpiling has helped suppress trade levels into Northern Ireland in the first week of the New Year, easing the switch, some gaps have already appeared on supermarket shelves.
One large food manufacturer with 15 trucks bound for Northern Ireland could not send them because they did not have customs declarations filled out, the committee heard.
Seamus Leheny, policy manager for Northern Ireland in the Logistics UK group, said the new customs demands were hitting companies throughout the supply chain.
"One operator sent 285 trucks to GB (Great Britain), they only got 100 of those back to Northern Ireland," he said. "The knock on effect is they can't service NI (Northern Ireland) exports going back to GB because they've got lorries and equipment sitting in England waiting for loads that aren't ready yet."
Similar problems have been detected on the busy cross-Channel border between Britain and France, and freight companies have said that a return to normal trade levels later this month will put a huge strain on the borders.
