Members of the US congress have called on Boris Johnson to scrap his proposals to ban future prosecutions related to the Northern Ireland Troubles.
In a letter to the British Prime Minister, the American politicians also called for the UK government to reaffirm its commitment to the Stormont House Agreement.
It was signed by 36 members of Congress, led by Brendan Boyle and Brian Fitzpatrick.
In the letter to Mr Johnson, the US legislators expressed concern that the proposed legacy laws would strain the British-Irish relationship and "cement widespread feelings" that justice is being denied.
In July, Northern Ireland Secretary Brandon Lewis announced plans for a statute of limitations which would end all prosecutions for Troubles incidents up to April 1998.
It would apply to military veterans as well as ex-paramilitaries.
The proposals, which Mr Johnson has previously said would allow Northern Ireland to "draw a line under the Troubles", would also end all legacy inquests and civil actions related to the conflict.
It has been widely condemned by campaigners on both sides of the Troubles and across the political divide.
The US politicians said it would be a "serious mistake" for the British government to renege on its commitments laid out under the Stormont House Agreement.
It would lead to "major setbacks" in the search for justice and reconciliation, they added.
They said they were "disappointed" that the UK Government plans to introduce new legislation that would modify the Agreement's legacy laws.
"To be clear, we strongly disapprove these proposals," they added.
"We believe that they would not only prevent a pathway to justice, but that they would also strip these families of their legal rights protected under European Law and the Good Friday Agreement."
Boris Johnson's office has been contacted for comment.