This suspicion has deepened with the suggestion that Megrahi had been forced to withdraw his appeal against conviction or be refused leave to return to Libya to see out his dying days.

An estimated 600 pages of evidence, much of which may prove Megrahi's innocence, has been withheld from the public.

The families of British victims of the bombing are understandably dismayed that, with the withdrawal of the appeal, the best chance to have the evidence exposed has faded.

Although we cannot know the exact details of what the withheld documents contain, there is enough evidence already in the public domain to raise serious questions over the Libyan's conviction and why he was fitted up for a crime he almost certainly did not commit.

In the immediate aftermath of the bombing, investigations centred on Iran, Syria and the Popular Front for the Liberation of Palestine General Command (PFLP-GC) - the secular resistance group founded by George Habash.

Three weeks before the Lockerbie bombing, German anti-terror officers arrested Marwan Khreesat, a known bomb-maker with links to the PFLP. He was released, apparently due to lack of evidence, but in April 1989 further raids discovered two more bombs designed by Khreesat, both of which were specifically made for targeting aircraft.

Among those arrested were a number of Palestinians with links to the PFLP-GC.

PFLP-GC leader Ahmed Jibril was at that time under the protection of Syria.

It has been suggested that Jibril was assigned the Lockerbie bombing by the Iranian regime in revenge for the shooting down of an Iranian Airbus passenger plane by the USS Vincennes earlier in 1988 with the loss of 290 lives.

During the German arrests, a Toshiba tape-recorder packed with Semtex was discovered. Pieces of a similar model recorder were discovered among the Lockerbie debris. One of the Palestinian group, Abu Talb, is known to have been in Malta shortly before the bombing.

Despite the large amount of evidence pointing to Iranian or Syrian involvement, these lines of inquiry abruptly ended, apparently with a phone call from George Bush Senior to Margaret Thatcher warning her to stop Scottish police digging.

It is argued that the pending invasion of Iraq required the support of Iran and Syria, both of whom had pariah status a year earlier, and that therefore exposing their involvement in the atrocity was not helpful.

A politically expedient scapegoat was sought.

Libya fitted neatly into the frame. It was commonly regarded as a terrorist state with a history of funding and supporting armed groups including the IRA and its leader Muammar Gaddafi had been branded a "mad dog" by Ronald Reagan.

Two years earlier the US had bombed Gaddafi's residence in Tripoli, killing his adopted daughter. There also remained the unresolved issue of the murder of PC Yvonne Fletcher at the Libyan embassy in London.

In addition, Libya was the only Middle Eastern state to openly support Iraq during the US-led invasion.

As this seismic shift in political alliances occurred, new evidence conveniently appeared, in the form of a fragment of circuitboard from a timing device, apparently discovered in the remote Scottish countryside after the bombing.

This fragment was reportedly traced by the FBI to a swiss manufacturer who sold timers to Libya.

In November 1991, indictments for murder were issued against Megrahi, who was head of security for Libyan Arab Airlines (LAA), and Lamin Khafilah Fhimah, Maltese station manager for LAA.

Crippling United Nations-imposed sanctions eventually forced Libya to hand over the two men in 1999 in exchange for the lifting of the embargo.

Libya also agreed to pay $2.7 billion in compensation to the victims of the bombings, although it has been argued that this was not, as has been claimed, an admission of guilt but rather a way of having the punitive sanctions removed.

The two accused men were handed over to Scottish custody at Camp Zeist in the Netherlands.

The trial at Camp Zeist, before a panel of three Scottish judges, was a farce.

Megrahi was convicted solely on the spurious and lucrative evidence of a Maltese shop-owner who claimed the Libyan had bought items of clothing from his store weeks prior to the bombing.

These items, which had been found in the wreckage of Pan Am Flight 103, had been traced back to the shop of Tony Gauci.

Megrahi's co-accused Lamin Khafilah Fhimah was acquitted on the same evidence after a so-called "Libyan defector," in the pay of the CIA, was wholly discredited.

Investigative reporter Paul Foot commented at the time: "His testimony was such a fantastic farrago of lies and fantasies that it was thrown out by the Scottish judges."

Gauci was also handsomely paid for his "evidence" to the court but even then gave a wildly inaccurate description of the man whom he claimed had bought the clothes from his shop.

In addition, little if any evidence was provided to explain how Megrahi had allegedly secreted a suitcase bomb on a feeder flight in Malta, which was then transferred twice through Frankfurt and Heathrow airports undetected.

The claim that an explosive device was physically handled through three airports without suspicions arising is ludicrous.

Police investigating the atrocity had even been told by a security guard at Heathrow that a Pan Am baggage storage area at the airport had been broken into on the night of the bombing.

This, one would have thought, should have set major alarm bells ringing and added massive weight to the long-argued hypothesis that the bomb was placed on the flight at Heathrow, not Malta.

This evidence, given in a sworn affidavit, was withheld from the trial.

At the time UN observer to the trial Dr Hans Kochler described it as "a spectacular miscarriage of justice."

In 2007 the Scottish Criminal Cases Review Commission referred Megrahi's case to the High Court of Justiciary. SCCRC chairman Rev Dr Graham Forbes said: "The commission is of the view, based upon our lengthy investigations, the new evidence we have found and other evidence which was not before the trial court, that the applicant may have suffered a miscarriage of justice."

The full SCCRC report into the Lockerbie bombing has never been disclosed, however.

With the dropping of his appeal by Megrahi this week, in a step strongly believed to have been forced upon him in exchange for his freedom, campaigners and the families of the victims fear that the report may now never be published.

The calls for a full public inquiry into the bombing have been dismissed by consecutive governments, both Tory and Labour.

The main argument against such an inquiry was that it would jeopardise the criminal investigation.

In truth, the "criminal investigation" ended with Megrahi's indictment in 1991. There has never been any attempt to arrest the real culprits.