A group of US Lockerbie relatives has become involved in a war of words with a leading British campaigner over his attempts to clear the Libyan convicted of the atrocity.

Dr Jim Swire's efforts on behalf of the late Abdelbaset al-Megrahi have been branded a "disgraceful and expensive campaign" by the Victims of Pam Am Flight 103 Inc in a letter to the chair of the Scottish Criminal Cases Review Commission (SCCRC).

The SCCRC has lodged court proceedings and are considering a joint bid by the Megrahi family and campaigners, including Dr Swire, whose daughter Flora was among the 270 victims of the disaster in December 1988.

The US families said: "We do not support this petition nor do we support the position of the UK family member, Dr Jim Swire and those with whom he stands on this matter.

"While Dr Swire is a family member of a victim of the bombing, he speaks for himself and not for the US families of victims.

"We believe that justice was done in the Scottish judgment and the appeal, and we believe that the Scottish judicial system is praiseworthy, despite the calumny visited upon it by Megrahi's supporters."

The letter concluded: "It will never really be "over", but it is past time for Dr Swire and the Megrahi supporters to end their disgraceful and expensive campaign."

In an email to The Herald, Dr Swire said he was "saddened" by their view, but pledged to continue fighting.

He added: "I sincerely hope that this public expression of exasperation from US relatives will be seen in Scotland simply as a reminder, if one were needed, of the desperate misery that terrorist atrocities like Lockerbie inject into our communities, and profoundly wish that a genuine determination to established the facts will settle the burden for all of us."

A Crown Office spokesman said: "We can confirm that we are aware of the position of the US families board and we are intending to lodge answers to the SCCRC petition by March 6. It is not appropriate to comment further."

Megrahi died of cancer in May 2012.