A group of Irish-American Congressmen have written to the British Prime Minister, Boris Johnson, warning that any weakening of the Good Friday Agreement or return to a hard border would make the prospect of a future US-UK trade deal highly unlikely.
The letter, which has been seen by RTÉ News, was sent by the Friends of Ireland Caucus, a bipartisan group which is co-chaired by the influential Irish-American Congressman, Richard Neal.
Congressman Neal is also the Chairman of the powerful House Ways and Means Committee, which would oversee any future trade deal between the US and the UK.
The letter wishes Mr Johnson well but states: "We want to make it clear that we will oppose any US-UK trade deal if the Good Friday Agreement is undermined."
It is the latest Brexit warning to come from the US Congress.
On Monday, the Senate Democratic Leader Chuck Schumer sent a letter to the US Secretary of State Mike Pompeo warning that he will strongly oppose any US-UK trade deal that undermines the Good Friday Agreement or facilitates a return to a hard border.
In the letter, Senator Schumer also told the Trump administration to stop over-promising an "unconditional and unrealistic" post-Brexit trade agreement with the United Kingdom.
Last week, the US Speaker of the House Nancy Pelosi reiterated her opposition to any trade deal that undermines the Good Friday Agreement warning there would be "no chance" of such a deal being passed by Congress.
Ms Pelosi was reiterating remarks she made when addressing the Dáil earlier this year as part of a visit to Ireland with a group of high-ranking US politicians.