LONDON, Sept 13 (Reuters) - Britain"s Brexit minister David Frost on Monday said that the European Union must move in negotiations over the trading arrangements in Northern Ireland or Britain may unilaterally suspend the so-called "protocol". Under the protocol, Britain agreed to leave some EU rules in place in Northern Ireland and accept checks on goods arriving from elsewhere in the United Kingdom, in order to preserve an open land border with EU member state Ireland. Britain has asked for "substantial and significant change" covering areas including the movement of goods into Northern Ireland, standards for goods and governance arrangements, and a treaty framework which is not policed by the European Court of Justice. read more Frost has warned of "cold mistrust" in relations with the EU if they do not move, but on Friday, Frost"s EU counterpart, Maros Sefcovic, rejected the idea of renegotiating the deal. read more Frost raised the prospect of triggering "Article 16" of the protocol, which allows either side to dispense with its terms if they are proving unexpectedly harmful. "They would be making a significant mistake if they thought that we were not ready to use Article 16 safeguards, if that were to be the only apparent way forward to deal with the situation in front of us," he said at the House of Lords. "If we are to avoid this situation, there needs to be a real negotiation between us and the EU." Britain last week said it planned to extend post-Brexit grace periods on some goods imports to Northern Ireland in a move designed to give London and Brussels more time for talks about trade with the province. read more Frost said there needed to be space for negotiations. "I don"t in fact take Commissioner Sefcovic"s words as a dismissal of our position, I take them as acknowledgement of it," Frost said. "But I also take it as a fairly clear indication that there is more to be done. So I do urge the EU to think again."