LONDON (AP) — The U.K.’s biggest supermarket chains warned Wednesday that food supplies in Northern Ireland face disruption because of new checks imposed by Britain’s departure from the European Union.

After photos emerged showing empty shelves, the chief executives of Tesco, Sainsbury’s, Asda, Iceland, Co-Op and Marks & Spencer wrote to the government saying there would be “significant disruption” unless urgent action was taken to fix an “unworkable” system.

Britain left the economic embrace of the 27-nation bloc, the final stage of Brexit, at the end of 2020. A trade deal that took effect Jan. 1 allows Britain and the EU to trade in goods without quotas or tariffs.

But that is a far cry from the seamless, hassle-free trade the U.K. enjoyed while it was part of the EU’s single market. Companies face new expense and red tape, including customs declarations and border checks.

Northern Ireland remains closely tied to the bloc’s economy under the Brexit divorce terms in order to protect its open border with EU member Ireland. That means there are new checks between Northern Ireland and the rest of the U.K., including customs declarations for goods and export health certificates for animal-based food products.

A three-month grace period means some measures will not be imposed until April 1, but firms already say they are staggering under the burden of new paperwork.

Ian Wright, chief executive of Britain’s Food and Drink Federation, told a committee of lawmakers that Northern Ireland was at risk of going “from color TV to black and white” in terms of food choice.

The British government said in a statement that “a new dedicated team in government has already been set up and will be working with supermarkets, the food industry and the Northern Ireland Executive to develop ways to streamline the movement of goods.”

While fears of huge queues of trucks forming at English Channel ports connecting Britain and the EU were unrealized during the quiet Christmas and New Year’s period, the government has warned of delays as the volume of traffic picks up. And other hurdles to smooth trade have emerged.

Wright said that for one major U.K. firm he knew the process of exporting to the EU, which used to take three hours, now took five days. He called on Britain and the EU to take additional steps to remove trade friction.

“If we do not see progress then costs will rise, choice will erode and quality will also dissipate, and that can’t be good for anybody,” he said.