LONDON (Reuters) - Britain’s leading supermarket groups have warned the government that “urgent intervention” is needed to prevent significant disruption to Northern Ireland food supplies in the coming months. Britain is no longer part of the European Union’s single market and customs union, but Northern Ireland has a foot in both camps as part of the UK’s customs territory while still aligned with the EU’s single market for goods. Under the Northern Irish protocol, which covers post-Brexit trade between Britain and Northern Ireland, supermarkets selling into the territory have a three-month grace period to adapt their supply systems to the new trading reality. Since the turn of the year, however, some supermarkets in Northern Ireland have had shortages of fresh goods usually imported from Britain because they have struggled to shift to new processes and bureaucratic procedures. The bosses of retailers including Tesco, Sainsbury’s, Asda and Marks & Spencer have written to Cabinet Office minister Michael Gove, warning that the situation could worsen. In the letter, seen by Reuters, they said it was essential a long-term solution is agreed with the EU before the grace period for simplified controls ends on March 31. “All our businesses and suppliers have invested significantly in the last few months to avoid disruption, but that will become inevitable if the proposals governing movement of food from Great Britain to Northern Ireland are adopted,” the letter said. “We recognise the European Commission needs to see increased compliance to support the concessions it granted through the Northern Ireland protocol, but the current proposals, increased bureaucracy and certification in such a short timescale are unworkable.” The chief executives called on Gove to create a dedicated working group to co-ordinate government agencies to integrate customs and food controls. “It also requires an open discussion with the EU explaining why we can’t accommodate changes to the current approach to transporting food to Northern Ireland but stressing we are working towards a robust system as quickly as possible,” the letter said, urging the government and EU to agree that more time is required to implement a new system. Gove said on Wednesday that a dedicated team has been set up and was working to find a solution. “We will make it clear to the European Commission what the consequences would be if supermarkets are not in a position (after March 31) to carry on with the service they provide to Northern Ireland consumers,” he told parliament. Prime Minister Boris Johnson said there were “teething problems” in trade between Britain and Northern Ireland but that goods were flowing effectively and in normal volumes. “But I can confirm ... that if there are problems that we believe are disproportionate, then we will have no hesitation in invoking Article 16,” he told parliament. Article 16 of the Northern Irish protocol details so-called safeguard measures that allow either party to take unilateral action if there is an unexpected negative effect arising from the agreement.