Julian Smith, Northern Ireland’s new Secretary of State, was sent a letter from the bipartisan committee in US set up to protect the Belfast Agreement. Photograph: Rebecca Black/PA Wire

April 17th, 2019: US speaker of the House of Representatives Nancy Pelosi addresses the Dáil, saying that there will be "no chance" of a US-UK trade deal should Brexit undermine the Belfast Agreement. Video: Oireachtas TV

A committee set up in the United States to protect the Belfast Agreement has warned the incoming Northern Ireland Secretary that a future US-UK trade deal will be “all but impossible” if the peace accord is undermined.

The bipartisan group, which includes five former US ambassadors, two former state governors and foreign policy experts including Nancy Soderberg, said it was “deeply concerned” at UK prime minister Boris Johnson’s rejection of the backstop last week.

“We remain deeply concerned given the new prime minister’s recent statement in the Commons that there can be no Irish backstop in the withdrawal agreement, even one with a time limit,” states the letter to Julian Smith that has been seen by The Irish Times. “We view the belief that alternative arrangements can easily solve the problem of the Irish Border with a healthy scepticism as do many experts.”

Noting house speaker Nancy Pelosi’s comments to The Irish Times last week that Congress will not sign off on a trade deal if the peace agreement is undermined, the letter states that a trade deal will be “all but impossible”.

Outlining the role played by the United States in the peace process more than 20 years ago, the letter notes that the US remains engaged and “deeply committed” to protecting and sustaining the core principles of the Good Friday Agreement.

“Whatever the future direction of the Brexit process we reiterate that the Irish Border must remain open and seamless so that we can maintain the peace that we all have worked so hard to achieve together.”

It also urges the new Northern Ireland Secretary to make the restoration of devolved governance at Stormont a “priority”.

Congressional approval The latest intervention by senior figures in the United States reflects increasing alarm by many in the Irish-American community about the political situation in Northern Ireland and the implications of Brexit on the future of the Border.

Ms Pelosi’s warning last week to the British government that there should be no return to a hard border on the island of Ireland, has again prompted doubts about the viability of a future trade deal between the US and Britain – a key priority for those advocating Britain’s departure from the EU.

Speaking following Ms Pelosi’s comments, Mr Trump said on Friday that he had spoken to Mr Johnson by phone and supported an “ambitious trade agreement” with Britain after Brexit. Britain was “actually impeded by the relationship with the European Union,” he said. “I think we can do three to four, five times what we’re doing.”

But any trade deal between Britain and the US – whether or not Mr Trump remains in the Oval Office after next year’s election – needs the sign-off of Congress.