BELFAST—The European Union rejected a British demand to renegotiate their deal governing the trading position of Northern Ireland, saying that to so would only bring instability and uncertainty. European Commission Vice-President Maros Sefcovic, who oversees EU relations with post-Brexit Britain, said on Friday that the Northern Ireland protocol needed to be properly implemented. “A renegotiation of the protocol—as the UK government is suggesting—would mean instability, uncertainty, and unpredictability in Northern Ireland,” he said in a speech at Queen’s University in Belfast. 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. The arrangement has effectively placed a border in the Irish Sea, angering pro-British unionists who believe it divides them from the rest of the United Kingdom and complicating the …