Former Labour MP Tam Dalyell and Edinburgh law professor Robert Black urged the Scottish and UK governments to answer reports there is evidence Abu Nidal was a US agent.
They have long believed Abu Nidal, who died in Iraq in 2002, and his Popular Front for the Liberation of Palestine General Command were responsible for co-ordinating the bomb that blew up Pan Am flight 103 over Lockerbie on 21 December, 1988 with the loss of 270 lives.
Intelligence reports, said to have been drawn up for Saddam Hussein's security services, said Kuwaitis had asked Abu Nidal, whose real name was Sabri al-Banna, to find out if al-Qaeda was present in Iraq.
The reports referred to Abu Nidal's "collusion with both the American and Kuwaiti intelligence apparatuses in co-ordination with Egyptian intelligence".
And campaigners said the latest evidence adds weight to the claims that Libyan secret agent Abdelbasset Ali Mohmed al-Megrahi – who was found guilty of the atrocity in 2001 – and the Libyan government were scapegoats to cover up a wider plot.
Mr Dalyell said the reports added weight to the theory that Lockerbie was a "tit-for-tat" attack for the shooting down of an Iranian passenger airliner by the warship USS Vincennes in 1988, and was allowed by the US administration.
He said the claims that Abu Nidal was working for the Americans would explain some of the mysteries that surrounded the Lockerbie outrage. These included a notice that went up at the American embassy in Moscow warning diplomats not to travel on Pan Am flights, and senior South African figures being "hauled off" the plane before its final flight.
The diplomats were replaced by students, who lost their lives.
Added to that is the mystery over why then prime minister Margaret Thatcher overruled her transport secretary, Cecil Parkinson, and stopped a public inquiry into the attack. It has been claimed this was because the US administration did not want an inquiry.
In a joint statement issued yesterday, Mr Dalyell and Prof Black said: "If the American public had known of a link with Abu Nidal, and had known that the US government knew enough to pull VIPs off the plane and let home-going students take their place, there would have been fury at a time of transition between the presidencies of Ronald Reagan and George Bush Snr.
"The fact that the Iraqi government either executed Abu Nidal or forced him to commit suicide suggests they had discovered he was an American spy."
Mr Dalyell and Prof Black – who with Lockerbie relative Dr Jim Swire persuaded the Libyan government to hand over Megrahi for trial – said they were "deeply and personally concerned" about the Libyan, who is suffering from cancer.