Former Iranian intelligence officer Abolghassem Mesbahi told a TV documentary, screened last night and repeated tonight, that the December 1988 bombing was undertaken by the Popular Front for the Liberation of Palestine-General Command (PFLP-GC) on behalf of Tehran.
He claimed it was carried out in revenge for the mistaken shooting down in July 1988 of an Iran Air Airbus by the USS Vincennes, a US Navy cruiser in the Gulf.
Iran's then leader Ayatollah Khomeini had pledged the skies would "rain blood" in revenge.
During Al Jazeera's "Lockerbie: what really happened?" documentary, Mr Mesbahi said: "Iran decided to retaliate as soon as possible. The decision was made by the whole system in Iran and confirmed by Ayatollah Khomeini.
"The target of the Iranian decision makers was to copy exactly what happened to the Iranian Airbus. Everything exactly same, minimum 290 people dead."
Campaigner Dr Jim Swire said yesterday that UK relatives of the victims, including himself, are preparing to apply to the Scottish Criminal Cases Review Commission (SCCRC) for the conviction of Libyan Abdelbaset al Megrahi to be overturned. Megrahi died in 2012 protesting his innocence.
Mr Swire, whose daughter Flora, 23, was on board Flight 103 said: "Some British relatives have decided that enough is enough and we will be applying within weeks for a further appeal against the Megrahi verdict. We have a right to know who killed our families and why the British Government and authorities responsible for the safety of the aircraft failed in their duty. We are not going away."
A successful SCCRC application could start the third appeal into Megrahi's conviction.
The Al Jazeera documentary fingered Ahmed Jibril, secretary-general of the PFLP-GC, as the key figure behind the attack, which was led by Hafez Dalkamoni. The bomb itself is alleged to have been made by Jordanin Marwan Khreesat.
In December 1988, an anonymous man took responsibility for the crash in the name of the Guardians of the Islamic Revolution and in retaliation for the downing of the Iran Air flight.
Earlier that year, German police raided a PFLP-GC cell and found Toshiba cassette recorder bombs, and arrested Khreesat and Dalkamoni. The bombs were later found to be almost identical to the one used to blow up Flight 103.
But during the Lockerbie investigation, the spotlight turned from Iran to Libya when forensic examination of the suitcase that carried the bomb found it had contained a Maltese-made babygrow. Maltese shopkeeper Tony Gauci identified Megrahi, a Libyan intelligence officer, as the man who bought the baby clothes and other items found in the suitcase.
In 1991 an indictment for murder was issued against Megrahi and Malta airport manager Al Amin Khalifa Fhimah. The pair were handed over to Scottish authorities in 1999. Fhimah was later cleared of any involvement in the bombing.
Egyptian-born Abu Talb had also previously emerged as a key suspect, with his accusers alleging he had smuggled the bomb onto the flight. However, when Megrahi went on trial in 2000, Talb was called as a prosecution witness.
New information about Talb and his activities in Europe was sent to the SCCRC during an earlier investigation. It claimed he was paid millions of dollars by Iran following the bombing. In 1989, a Swedish court convicted Talb of bombing a synagogue in Denmark and gave him a life sentence. He has since been released.
John Ashton, Megrahi's biographer, said: "There is very little that is new here. For about 18 months after the bombing Tehran got the blame, before attention focused on Libya. I am glad [Al Jazeera's] report has put the focus back on Iran."
A Crown Office spokesman said: "Mesbahi's claim that Iran was responsible was first reported in the media in the late 1990s and was available to the defence before the trial but they did not call him as a witness.
"The wider alleged involvement of the PFLP-GC has been repeatedly reported over many years but was addressed in full and rejected at the original trial."
He added that Megrahi was convicted and that the verdict was upheld following an appeal.