Marks & Spencer estimates that up to 1,000 product lines in Northern Ireland could be impacted when the Brexit transition comes to an end - around 10% of its food range.
The upmarket grocer doesn't know if it will legally be able to transport sausages, mincemeat and breaded poultry to Northern Ireland, or whether its organic and free-range foods will be able to be sold as such after the UK's transition period out of the EU finally comes to an end on January 1.
In a statement, Steve Rowe, the CEO of M&S, told ITV News: "We are committed to Northern Ireland and our priority is to make sure we continue to deliver the same choice and great quality range that our loyal customers enjoy there now.
"We have extensive contingency plans in place but the clock is ticking and if clarity is not given soon there is a real risk to supply from the UK mainland, which could limit customer choice in Northern Ireland."
Last Friday, the prime minister hosted a call with the chief executives of 11 of the UK's biggest retailers, including Tesco, Sainsbury's, Asda, Morrisons, M&S, the Co-op, Aldi, Waitrose, Lidl, Iceland and Ocado.
During the meeting Prime Minister Boris Johnson sought to address their fears that the implementation of the Northern Ireland Protocol could lead to gaps on the shelves.
"Nothing can stop the Great British sausage making it to Belfast", an upbeat Mr Johnson told the retailers.
The retailers' concerns emerged as the European Union yesterday issued one of its most positive assessments of post-Brexit trade negotiations.
EU Commission president Ursula von der Leyen said: "We've seen in the last days, better progress, more movement on important files."
She said because legal texts had made such progress, further discussions by video could progress too, "since there is now substance where you can go through line by line".
Over the past few months, the talks have failed to make much headway on some key issues, notably over fishing rights, business regulations and state aid.
"Progress, for example, has been made on the question of state aid," Ms von der Leyen said over a key EU demand that the UK does not excessively subsidise products that could undercut local EU products.
Prime Minister Boris Johnson's spokesman, Jamie Davies, said negotiations will continue next week, well past the mid-November date previously seen as a deadline.
He said talks would continue remotely and resume in person "as soon as it's safe to do so".
"We will continue to work hard to reach an agreement and we will continue that work today and next week," he said.