A major parcel courier has paused some delivery services into Europe - including Ireland - because of pressure caused by new post-Brexit red tape.
DPD became the latest company to warn that Boris Johnson's divorce settlement with Brussels had led to more complex processes at the border.
Seafood exporters said they have been hit by a "perfect storm" of bureaucracy, IT problems and confusion following Brexit.
Marks & Spencer said the new rules and regulations are set to "significantly impact" its overseas ventures in Ireland, the Czech Republic and France.
And hauliers described being "overwhelmed" by red tape due to new checks on deliveries to Northern Ireland from the rest of the UK.
DPD said that up to 20pc of parcels had incorrect or incomplete data, meaning they had to be returned to customers, and announced a pause to its road service into Europe and Ireland until Wednesday.
The company said in a statement: "The EU-UK Trade and Cooperation Agreement resulted in more complex processes, and additional customs data requirements for parcels destined for Europe.
"This, along with delays and congestion at UK ports for channel crossings, has placed extra pressure on our turnaround and transit times.
"We are seeing up to 20pc of parcels with incorrect or incomplete data attached, resulting in these parcels needing to be returned to customers, so that the required data can be provided.
"In view of this unprecedented set of circumstances we believe that it is only right to pause and review our road service into Europe, including the Republic of Ireland. During this time, we will work with our customers to validate and correct the data we have in our system, to reduce the delays and enable us to resume normal service.
"This pause in our operation will be as short as possible and we intend to recommence this service on Wednesday January 13."
In a statement, An Post said that it is trading “fully and smoothly with almost all of the UK online retailers” for its deliveries across Ireland.
It said that while there are “some delays” which should be expected due to UK retailers “bedding in new data requirements” before shipping, the majority of items dispatched through An Post systems are “are released for immediate delivery on arrival into Ireland.”
“An Post has put in place a digital solution for all its UK retailers which simplifies the process and enables seamless dispatch and immediate delivery. 95pc of An Post’s UK Retailer customers have opted to use this digital system,” said a spokesperson.
They added that An Post is “helping” its UK retailers to “iron out” any issues relating to Brexit.
Donna Fordyce, chief executive at Seafood Scotland, said exporters faced "new bureaucratic non-tariff barriers" with no one body able to fix the situation.
"It's a perfect storm for Scottish seafood exporters. Weakened by Covid-19, and the closure of the French border before Christmas, the end of the Brexit transition period has unleashed layer upon layer of administrative problems, resulting in queues, border refusals and utter confusion," she said.
"IT problems in France meant consignments were diverted from Boulogne sur Mer to Dunkirk, which was unprepared as it wasn't supposed to be at the export front line. There have also been HMRC IT issues on the UK side that need to resolved ASAP regarding certification.
"A lack of knowledge and understanding of the required paperwork means some companies are ill-prepared for the new checks, which are taking far longer because of the mistakes being uncovered. When the systems settle down, checks should be carried out on samples from each load but now entire consignments are having to be checked to satisfy requirements."
M&S boss Steve Rowe warned that the trade agreement between the UK and the EU is causing problems with "potential tariffs on part of our range exported to the EU, together with very complex administrative processes".
Haulier industry body Logistics UK said deliveries were being delayed as lorries arrived in Belfast with incomplete paperwork.
In order to avoid a hard border with the Republic, Mr Johnson agreed Northern Ireland would remain in the EU single market - but that has meant checks on goods arriving from Great Britain.
Labour accused the Government of failing to properly prepare for the end of the transition period on December 31.
Shadow transport secretary Jim McMahon said: "The Government promised it had a plan to make sure things ran smoothly for businesses and hauliers post-Brexit.
"It's clear the problems caused by its poor preparation and delaying tactics have not gone away. Ministers have to get a grip on this and make sure essential workers are actually able to do their jobs, or we risk seeing a repeat of the chaos on our roads at Christmas."