The US president claimed Irish unification was inevitable.
Relations between Britain and America have been further strained after Donald Trump expressed support for a united Ireland.
The US president claimed Irish unification was inevitable as he spoke to reporters alongside Taoiseach Micheál Martin, adding it “may as well happen now”.
Answering a question from the Press Association at the start of his two-day visit to Ireland, Mr Trump said: “I’d love to see it unified. I think it would be a great feather in everybody’s cap if that happened. It’s going to happen eventually.”
In response, Downing Street pointed to comments from Prime Minister Andy Burnham earlier in the week rejecting the prospect of an immediate border poll.
Mr Trump’s remarks brought condemnation from unionist politicians in Northern Ireland.
DUP leader Gavin Robinson said Northern Ireland’s future would be decided by its people, not “commentary in London, Dublin or Washington”.
Traditional Unionist Voice leader Jim Allister said: “Northern Ireland is not a pawn in Trump’s fallout with the UK.”
But Sinn Féin leader Mary Lou McDonald said the president’s comments were “timely” and Mr Martin “must now begin planning for a united Ireland”.
In England, Conservative leader Kemi Badenoch accused the president of “shooting from the hip”, adding: “This sort of loose talk about the future of democratic countries is deeply unhelpful and creates unnecessary tension.”
Mrs Badenoch also called for “clarity” from Mr Burnham in defence of the union.
The UK-Ireland border remains a sensitive issue, with Mr Burnham triggering a political row earlier in the year after saying a referendum on reunification was “off the table”.
Under the terms of the 1998 Belfast/Good Friday Agreement, the UK’s Northern Ireland secretary can decide to hold a border poll, and has a duty to call one if it appears likely a majority would vote for Irish unification.
The people of the Republic of Ireland must also vote in favour to effect constitutional change.
Ireland’s deputy premier and leader of the Fine Gael Party, Simon Harris, said the comments show “how much the conversation about a unified island is advancing at home and abroad” as he urged political parties to engage in a “constructive, informed and evidence-based way”.
Mr Trump’s comments come amid already strained relations between London and Washington over issues including the war in Iran, sanctions on illegal Israeli settlements in the West Bank and the president’s demand earlier in the year to annex Greenland.
In another sign of difficult UK-US relations, Mr Trump also cast doubt on Britain’s willingness to defend the Falkland Islands should Argentina attempt an invasion.
