The Crown Office has used the 26th anniversary of the Lockerbie bombing to proclaim the safety of the conviction of Abdelbaset al Megrahi, the only man so far convicted of the bombing.

The department briefed yesterday that a review of the case had "confirmed beyond doubt" the Libyan's guilt, while today its head, Lord Advocate Frank Mulholland QC, has personally reaffirmed that guilt.

Mulholland has been unusually vigorous in denouncing Megrahi's supporters, who include relatives of the Lockerbie dead, branding them "conspiracy theorists" two years ago. It is hard to imagine his opposite number in England and Wales, the director of public prosecutions, taking to the media to defend a conviction and take on critics. But while this strident tone has raised eyebrows, Mulholland's statements are more notable for ignoring four large elephants in the middle of his legal chambers.

The first is the ongoing review of the case by the Scottish Criminal Cases Review Commission (SCCRC), the statutory body that has the power to refer convictions to the appeal court. As Mulholland well knows, a previous review by the commission referred the case on no fewer than six grounds. The terminally ill Megrahi abandoned the resulting appeal to improve his chances of being granted compassionate release, but was confident that his name would one day be cleared. Remarkably, one of the six grounds was that the three Scottish law lords who convicted him had made a fundamental error of judgment when they found that the clothes incriminating Megrahi had been bought on December 7. In doing so, the commission, in the eyes of some, came as close as it legally could to saying that the guilty verdict was itself wrong.

More seriously for the Crown Office, four of the other grounds concerned its failure to disclose important evidence to Megrahi's defence team. This included evidence that the Crown's star witness, Maltese shopkeeper Tony Gauci, had expressed an interest in receiving a substantial reward and was under the strong influence of his brother Paul, who regularly nagged the police about being rewarded. The SCCRC discovered Gauci was later secretly paid $2 million by the US Department of Justice, and his brother Paul $1m.

When, in 2012, this ­newspaper published a leaked copy of the SCCRC's 800-page review, the Crown Office went into panic mode, anonymously briefing a Scottish tabloid that Megrahi's case had "more holes than a piece of Swiss cheese" then issuing a press statement that significantly downplayed the commission's findings.

The second elephant is the two-year-old police investigation, led by Police Scotland's Deputy Chief Constable Iain Livingstone, into criminal allegations made against some of those originally involved in the inquiry by the committee of the Justice for Megrahi group.

When the allegations were first made to the then Justice Secretary Kenny MacAskill, the Crown Office immediately denounced them as groundless, despite not having seen the detailed dossier of evidence assembled by the committee. Many were shocked by the intervention, believing it might compromise the police inquiry and that it raised serious questions about Mulholland's independence as the chief public prosecutor. Unfortunately for the Crown Office, the police clearly do not share its contempt for the allegations. If the investigation concludes there was no criminal misconduct, the Crown Office still has to explain why it failed to disclose so much important evidence. In the view of its critics, notably Dr Jim Swire, who lost his daughter in the bombing, the matter must be addressed in a public inquiry - something successive Scottish governments have been reluctant to grant.

The third elephant is forensic evidence concerning a small fragment of electronic circuit board, recovered from an item of clothing that was supposedly in the same suitcase as the bomb. According to the prosecution, it matched boards in timers supplied to Libya by a Swiss firm called Mebo, which shared offices with a Libyan company part-owned by Megrahi.

Evidence uncovered prior to Megrahi's abandoned appeal demonstrated that the fragment could not have originated from one of the Libyan timer boards. The discovery has fuelled claims the fragment was a plant, which has in turn encouraged the Crown Office to call its opponents conspiracy theorists. However, as Mulholland must be aware, the breaking of the link between the fragment and the Libyan timers leaves the prosecution case in shreds, regardless of whether it was planted.

The fourth elephant is the lack of evidence from Libya to implicate either Megrahi or the Gaddafi regime in the bombing. During the country's 2011 revolution, senior officials, keen to curry favour with the West, lined up to accuse the regime of sponsoring the attack.

The best known of them, the head of the National Transitional Council and former justice minister Mustafa Abdel Jalil, claimed to have proof that Gaddafi ordered the bombing.

All this must have been music to the Crown Office's ears, but, when pushed to reveal his proof of the regime's guilt, the best Jalil could offer was that it had funded ­Megrahi's legal case.

Sadly, Libya has become too dangerous for the Scottish police to conduct investigations there. Even if it were not, they would likely find the cupboard was bare. In the four years since the revolution, ­nothing has emerged publicly from the ruins of the old regime to affirm Megrahi's guilt, let alone Libya's.

No doubt Mulholland's public declarations will continue to ignore the four elephants in his legal chambers, but he must knows that their ever-fiercer stamping may one day bring Megrahi's conviction crashing around his ears.

John Ashton is the author of the authorised ­biography of Abdelbaset al Megrahi, Megrahi: You are my Jury, (Birlinn, 2012) and Scotland's Shame: Why Lockerbie Still Matters (Birlinn, 2014). From 2006-09, he worked as a researcher with Megrahi's legal team.