LONDON, Jan 13 (Reuters) - Britain’s major supermarkets have warned the government that “urgent intervention” is required to prevent significant disruption to Northern Ireland food supplies in coming months. Britain is no longer part of the European Union’s single market and customs union, but the British-run region of Northern Ireland has a foot in both camps – part of the UK’s customs territory but also still aligned with the EU’s single market for goods. Despite a three-month grace period on some new rules, some supermarket shelves in Northern Ireland have been empty since the turn of the year as firms struggle to import fresh goods from Britain. The bosses of supermarket groups including Tesco, Sainsbury’s, Asda and Marks & Spencer, have written to Cabinet Office minister Michael Gove to raise the issue. In the letter, seen by Reuters, they note that the current grace period for simplified controls on sending food to Northern Ireland ends on March 31 and said it was essential a long-term solution is agreed with the EU before that. “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,” it 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 supermarket CEOs called on Gove to create a dedicated working group to co-ordinate government agencies to integrate customs and food controls. And they want the government to agree with the EU that more time is required to implement a new system. “We accept that clear progress needs to be made by (April 1)... However, until then, we need an assurance that the current process based on a trusted trader scheme ... doesn’t change until a workable replacement is agreed,” the letter said.