Paula and Catherine McCartney said the man was being protected

Catherine and Paula McCartney made the claim in an interview with the BBC's Newsnight programme.

Mr McCartney, 33, was fatally stabbed outside a bar in Belfast city centre in January.

"The person who we believe ordered it is high up in the IRA: it's all to protect him," said Catherine.

"This person is bigger than the IRA, he's bigger than the whole movement."

Paula added: "He's being allowed to be bigger than the whole movement."

Mr McCartney's family claim they have been intimidated by the IRA.

Earlier this week, the two sisters met Prime Minister Tony Blair formally for the first time to update him on their campaign for justice.

They said they wanted to dispel a "misconception" that those involved in Mr McCartney's murder were "rogue elements" in republicanism.

They told Mr Blair that they felt Sinn Fein was not doing all it could to help them.

In March, they had separate meetings with Senator Ted Kennedy and President George Bush, both of whom refused to meet Sinn Fein President Gerry Adams at the time.

Two people have been charged in relation with Mr McCartney's murder.