Retail bosses have warned the sector faces a "perfect storm" as new border regulations come into force in April with a raft of retailers still unable to serve customers in Northern Ireland.
Trade bodies and firms have called on the Government to extend the current grace period amid continued uncertainty over the level of checks which will then be introduced.
It comes as a number of major UK retailers - including Halfords, John Lewis, Fortnum & Mason and AO - remain unable to deliver products to customers in Northern Ireland, more than six weeks after the Brexit deal was agreed.
Some retailers have said they are still developing and finalising IT systems which can deal with new trade requirements for Northern Ireland.
However, Aodhan Connolly, director for Northern Ireland at the British Retail Consortium (BRC), warned that the situation could become even more complicated in the coming months.
He told the PA news agency that the IT systems being installed by retailers in order to fulfil restrictions put in place at the start of January, could need a complete overhaul just three months later.
"We just don't know at all - it is entirely possible the systems they have put in place will not be what's needed in April," he said.
"We need time. In an ideal world we would have an extension and retailers would have more time to prepare for what needs to be a long-term sustainable solution that allows us to continue giving Northern Irish households choice and affordability.
"Retailers have been making the situation work because they are bending over backwards to make sure they can serve people here.
"But they know that change is coming in April, and then we could have a perfect storm.
"New rules on parcels and products of animal origin will come in as well pressure on the supply chain as hospitality will be reopening, schools will be re-opening and we have the change of seasons for goods."
Retailers have also expressed their frustration with Government over the complexity of current regulations and the short-notice nature of its introduction.
One UK business, which asked not be named, criticised suggestions that there is nothing preventing the transport of parcels to Northern Ireland.
"It cannot be a surprise to HMRC to discover that large businesses plan their technology programmes months in advance and that these cannot simply be switched on or off instantaneously."
The BRC said that parcel regulations were only confirmed 18 hours before they came into force.
Another unnamed retailer accused the Government of "moving the goalposts" just days before the UK officially left the EU.
"We had been advised that our system just needed to show product codes but were told we needed more info on December 28 or 29," it added.
"The development of IT systems can't be done in days so sadly there has been this disruption."
Mr Connolly said the Government rolled out its customs declarations system in mid December, its trader support service on December 21, and parcel regulations at the very end of the month.
Online furniture retailer Made.com is another company which has said it is still currently unable to operate deliveries in Northern Ireland due "to the new customs processes that came into place as part of the Brexit agreement".
A UK Government spokesperson said: "We continue to work closely with businesses to support the implementation of the new arrangements.
"We have been clear that there are outstanding problems with the Protocol that need to be resolved to ensure trade continues to flow smoothly and there is as little impact as possible on the everyday life of communities.
"We held talks with the EU this week and agreed to intensify work to address all outstanding issues, with the shared objective to find workable solutions on the ground."