DUBLIN/LONDON (Reuters) - The European Union on Wednesday rebuffed a British demand to extend a grace period for checks on goods going from Britain to Northern Ireland, saying the post-Brexit trade treaty gave London enough tools to solve the problems. But it agreed to “work intensively” with Britain to resolve difficulties that have already impeded deliveries of goods, notably food, from other parts of the United Kingdom and caused shortages in supermarkets, even with a grace period still in force. The dispute, which stems from Britain’s exit from the EU’s orbit on Jan. 1, threatens to reopen a rift that bedevilled years of Brexit talks, and further strain relations between the EU and its former member. British minister Michael Gove sent a terse letter to European Commission Vice President Maros Sefcovic, demanding that grace periods for the transport of food from Britain to Northern Ireland be extended from a few months to at least two years. Then, after a video call with Gove, Sefcovic told Irish RTE television that, under the terms of the post-Brexit trade deal signed in December, it was for Britain to resolve the problems. “I really think that if all the flexibilities we put on the table and into the (Northern Ireland) Protocol would be used to the maximum, that all of the issues which we are discussing today would be really resolved,” Sefcovic said. Another politician present at the meeting, Northern Ireland Deputy First Minister Michelle O’Neill, said Sefcovic had told Gove the EU “expects rigorous implementation of the Protocol”. In a statement, Gove said he and Sefcovic had reiterated their full commitment to the agreement that made peace between the province’s pro-British and pro-Irish communities two decades ago, and to the proper implementation of the Protocol, and would meet in London next week. CUSTOMS BORDER Britain left the EU’s single market a month ago with a trade deal that also created a customs border between Britain and Northern Ireland. As part of its divorce treaty, it had agreed to ensure there were no checks on goods crossing the land border with the Irish Republic by instead introducing checks on goods reaching Northern Ireland from other parts of the UK. This was intended to protect peace in the province while also preventing it being used as a back door into the EU’s single market. The issue was arguably the most contentious in Britain’s five-year Brexit negotiations, and the search for an arrangement led to the downfall of Prime Minister Boris Johnson’s predecessor, Theresa May. The rules are due to get even tighter when a three-month grace period for some goods expires. In his letter to Sefcovic, Gove said the grace period “must be extended until at least 1 January 2023”. “If it is not possible to agree a way forward in the way we propose, then the UK will consider using all instruments at its disposal,” he added, saying that what was needed were “political, not technical, solutions”. London feels it has gained some moral authority since the European Commission briefly threatened last week to impose emergency controls on vaccines crossing the land border. After an outcry from London, Belfast and Dublin, the commission swiftly changed course, but the blunder reinforced Britain’s case that the agreement needs updating.