Leading Irish-American politician Ted Kennedy says he will not meet Gerry Adams during the Sinn Fein leader's St Patrick's Day trip to the US.
Mr Adams is facing tough questions in the US
A spokeswoman for Senator Kennedy said he had cancelled a meeting because of the IRA's "ongoing criminal activity".
The move comes as Sinn Fein faces increasing pressure over IRA involvement in the killing of Robert McCartney in Belfast.
Mr Adams said he was "disappointed" and Mr Kennedy had been "badly advised".
He said his party was committed to the peace process and was confident they would work with Mr Kennedy in the future.
Mr Kennedy and Mr Adams had been due to meet on Thursday.
Mr Adams has also been denied a meeting with President Bush at the White House. Instead, Mr Bush will play host to Mr McCartney's sisters.
Mr McCartney, 33, was stabbed to death after a row in a bar near the city centre on 30 January.
Mr Adams will face tough questions in the US over the killing and over alleged IRA involvement in the £26.5m Northern Bank raid.
Kennedy spokeswoman Melissa Wagoner said: "Senator Kennedy has decided to decline to meet with Gerry Adams, given the IRA's ongoing criminal activity and contempt for the rule of law."
She said the events surrounding the death of Mr McCartney underscored the need for IRA violence and criminality to stop and for Sinn Fein to co-operate with police.
Meanwhile, Sinn Fein's most high profile supporter in Congress, New York Republican Congressman Peter King, called on the IRA to disband.
Mr King said the IRA had made a series of poor decisions that had sparked anger in Irish-American circles and was now standing in the way of a power sharing deal between Sinn Fein and Ian Paisley's Democratic Unionists.
He added that Americans were finding it "hard to see what the justification is for the continued existence of the IRA".
BBC Pentagon correspondent Adam Brooks said it showed the Sinn Fein leader was no longer assured of a warm welcome in the US.
And Mr Bush's decision to host the McCartney sisters and not him on St Patrick's Day was "a powerful signal from the president himself that America's patience with Sinn Fein is leaking away", he said.