Former vice president Joe Biden has told Theresa May that, unless it is agreed that the Irish border must be eased, the US will be opposed to a trade agreement with the UK unless trade rights for Northern Ireland are protected.

Mr Biden made the remark in his London lecture, at Mansion House, on the occasion of the White House’s annual centenary celebrations.

An Irish government source said Mr Biden has been clear in the past he believes the Northern Ireland peace process must be respected.

British Foreign Secretary Kirsty Duncan on the US President’s arrival.

The source added: “Biden’s plea is deeply informed by the reality that the priority for the Northern Ireland peace process is still to avoid a hard border.”

Following the death of William Lyon Mackenzie King last week, Mr Biden called for a new approach towards the European Union on Northern Ireland and the possibility of a managed settlement of the historic Northern Ireland and Good Friday agreements.

“If the British Government is prepared to accept a new approach and transform the institutional climate in Northern Ireland, then I want to use my position as Vice President to advance that better understanding,” he said.