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© Kevin Lamarque/Reuters
Donald Trump with Boris Johnson in 2017. Trump has promised the two countries could strike 'a very substantial trade agreement'.
Any future US-UK trade deal would almost certainly be blocked by the US Congress if Brexit affects the Irish border and jeopardises peace in Northern Ireland, congressional leaders and diplomats have warned.

Boris Johnson has presented a trade deal with the US as a way of offsetting the economic costs of leaving the EU, and Donald Trump promised the two countries could strike "a very substantial trade agreement" that would increase trade "four or five times".

Trump, however, would not be able to push an agreement through a hostile Congress, where there would be strong bipartisan opposition to any UK trade deal in the event of a threat to the 1998 Good Friday agreement, and to the open border between Northern Ireland and the Republic.

The comments came as Johnson was in Northern Ireland in an effort to revive power-sharing talks between his allies in the Democratic Unionist party and Sinn Féin, as well as discuss Brexit preparations.

Johnson's rise to power, and his demand for the EU to drop the backstop, which is intended to safeguard the open border after Brexit, has galvanised determination in Congress to make a stand in defence of the landmark accord, to which the US is guarantor.

"The American dimension to the Good Friday agreement is indispensable," said Richard Neal, who is co-chair of the 54-strong Friends of Ireland caucus in Congress, and also chairs the powerful House ways and means committee, with the power to hold up a trade deal indefinitely.

"We oversee all trade agreements as part of our tax jurisdiction," Neal, a Democratic congressman from Massachusetts, said in a phone interview. He pointed out that such a complex trade deal could take four or five years, even without the Northern Ireland issue.

"I would have little enthusiasm for entertaining a bilateral trade agreement with the UK, if they were to jeopardise the agreement."

Pete King, the Republican co-chair of the Friends of Ireland group, said the threat to abandon the backstop and endanger the open border was a "needless provocation", adding that his party would have no compunction about defying Trump over the issue.

"I would think anyone who has a strong belief in Northern Ireland and the Good Friday agreement the open border would certainly be willing to go against the president," King said.

In the event of a hard Brexit, in the absence of guarantees for the Northern Ireland agreement, the strength of sentiment among Irish Americans - a tenth of the population, many of them in swing states - could make it an issue in next year's presidential and congressional elections.

Johnson has refused to meet EU leaders until the backstop is scrapped. On Tuesday, Ireland's taoiseach, Leo Varadkar, told Johnson the backstop could not be removed from the UK withdrawal agreement.

After a contentious phone call between the two leaders, a spokesman for Varadkar said that alternatives to the backstop, as a means of guaranteeing the Northern Irish peace agreement "have yet to be identified and demonstrated".

For the past eight months, Congress has held up ratification of a new trade agreement with Mexico and Canada, the USMCA, which Trump has presented as an extraordinary achievement (though it differs little from its predecessor, Nafta). Representative King said a UK trade deal would face even greater obstacles.

"First of all trade deals are always difficult," the New York Republican said in a telephone interview. "There's any number of other labour and environmental issues that get brought up. But to have a solid block on one particular issue would make it very, very difficult to get it through Congress, unless the border issue is resolved."

The Democratic speaker of the House of Representatives, Nancy Pelosi, has said that a US-UK trade deal has "no chance whatsoever" of passing in Congress. Over the weekend, a committee of former members of Congress and foreign policy officials said "all of Irish America will support the Speaker right down the line".

The adhoc committee to protect the Good Friday agreement, established earlier this year, wrote to the UK's new secretary for Northern Ireland, Julian Smith, on Sunday to raise its concerns about Johnson's statements about abandoning the backstop.

A European diplomat in Washington predicted the Irish American caucus would be decisive in holding up an agreement. "I think there is enough meat in the Irish-American lobby to stop a UK trade deal if the Good Friday Agreement is affected," the diplomat said.

The Irish embassy has been energetically lobbying in defence of the 1998 peace agreement. The ambassador, Daniel Mulhall, said he has been pushing at an open door.

"There is a genuine groundswell of opinion within Irish America in favour of the Good Friday agreement and against anything that would be perceived to undermine that agreement," Mulhall said.

"Wherever I go, wherever I speak to Irish-American audiences, the first question is always to do with Brexit," the ambassador added. "And they always reflect a deep concern about Brexit."

"Politically we have a good caucus here. It's active ... They see the Good Friday agreement and all that's flowed from it as an achievement for Irish America .. and they're loathe to see that jeopardised in the Brexit context."

Amanda Sloat, a former state department official and now a Brexit expert at the Brookings Institution in Washington, said: "Trade deals are always challenging to ratify in Congress ... There will be significant resistance, as Speaker Pelosi has said, to ratifying a trade agreement that is seen to harm the Good Friday agreement or the interests of people in Northern Ireland."