The US president-elect, Joe Biden, has repeated his call for the border between Ireland and the United Kingdom to remain open as the contentious issue threatens to complicate the final stages of the Brexit process. Biden had stressed the importance of protecting Northern Ireland’s peace deal in the Brexit process in a call with the UK’s prime minister, Boris Johnson, after the Democrat won the US election against Donald Trump. He also said that he had discussed the issue with other European leaders. Johnson’s government is seeking a trade deal with the European Union but says it is willing to leave without one. That could complicate the situation at the sensitive Northern Irish border with Ireland – the UK’s only land border with the EU. Biden told journalists in Wilmington, Delaware, on Tuesday that the border must be open. “We do not want a guarded border,” he said, answering a question from a reporter on what he would say to Brexit negotiators. The 1998 Good Friday peace deal helped to end Northern Ireland’s 30 years of sectarian violence and created institutions for cross-border cooperation on the island of Ireland. Johnson put forward legislation in September that would break the Northern Ireland protocol of the Brexit divorce treaty that seeks to avoid a physical customs border between the British province and EU-member Ireland. Biden, who has talked about the importance of his Irish heritage, warned months ago as the Democratic presidential candidate that the UK must honour the 1998 agreement as it withdraws from the bloc or there can be no separate US trade deal. Johnson has never met Biden and commentators have suggested the prime minister will have to work hard to foster the “special relationship” between the historic allies. Ireland’s prime minister said on Monday he hoped the outline of a Brexit free-trade deal would emerge by the end of this week, despite what the EU negotiator called “fundamental divergences” at talks.