The man convicted of the Lockerbie bombing is not guilty as charged and there is no convincing argument for Malta's involvement in the terrorist act, according to the United Nations' appointed monitor of the trial in the Netherlands.

Hans Koechler, who was handpicked by the then UN Secretary-General Kofi Annan to monitor proceedings, told The Sunday Times: "I never really understood why the government of Malta did so little to reject these allegations and to defend the integrity of the country's civil aviation system."

Twenty years after the bombing, the government has gone no further than saying that it is monitoring proceedings of the second appeal. Air Malta did not comment.

Malta was implicated in the terrorist act because the prosecution had argued that Abdel Basset Al-Megrahi and Al-Amin Khalifa Fahima had placed the bomb on an Air Malta aircraft before it was transferred at Frankfurt airport on board the doomed Pan Am flight 103A.

The flight went to London Heathrow and was bound for New York's JFK airport before exploding over Lockerbie in Scotland an hour into the journey on December 21, 1988. All 259 people on board died as well as 11 locals on the ground.

The trial at Camp Zeist in the Netherlands had led the Scottish judges to conclude in 2001 that Mr Al-Megrahi was guilty. He was jailed for life while the other defendant was released.

In his report after the verdict, Dr Koechler had concluded that a miscarriage of justice may have occurred. Several years on, he stands by his conclusions: "The court did not come up with any convincing argument that Mr Al-Megrahi is the one who bought the clothes at the shop in Malta and that the 'bomb suitcase' was loaded at Luqa Airport."

Dr Koechler expressed doubt that Mr Al-Megrahi's ongoing appeal, which started on April 28, could be fair and impartial because of the "outright interference of the British government trying to withhold certain sensitive evidence from the defence".

He said political expediency had guided the original verdict, saying it reflected the political considerations related to the foreign policy interests of the involved states at that time.

One of Malta's leading lawyers, who had formed part of the legal team in the defence of the two Libyan suspects, also believes Mr Al-Megrahi is innocent.

Emmanuel Mallia told The Sunday Times: "I personally know the accused and have always firmly believed in his innocence."

Mr Al-Megrahi's appeal was ordered by the Scottish Criminal Cases Review Commission in 2007, after a four-year investigation came to the conclusion that a "miscarriage of justice" may have occurred.

Dr Mallia would not enter into the merits of the case because it is still sub judice. But he said his personal view was that the verdict was flawed.

"Having examined the judgment of the court at Camp Zeist and being aware of the salient evidence produced in the case by the prosecution, I feel that the evidence could never have amounted to guilt of the accusation according to law," Dr Mallia said.

He said the prosecution lacked reliable evidence that could prove guilt beyond reasonable doubt: "Although there were a lot of issues which could give rise to suspicion, anything argued on the basis of suspicion may lead to conjecture but not proof. Even if considering circumstantial evidence, we know that such evidence can mislead and, in order to rely upon it, it has to lead to one direction."

Some argue that at the early investigation stage Malta was perhaps too compliant. "The government gave access to the Scottish and American investigators to interview people and take any action deemed necessary. Some have argued that things may have been done differently with the Malta police having more direct control of the investigation".

A former Scottish judge regarded as the architect of the Lockerbie trial, Robert Black, also told The Sunday Times last week that there was never any evidence that the bomb left from Malta.

On his blog this week, Prof. Black contested arguments made by the prosecution at the Court of Criminal Appeal in recent days that Mr Al-Megrahi's trip to Malta with a false passport the day the bomb was planted, and his departure the day after, was a link to the commission of the offence.

"As regards the coded - not false - passport, it is of relevance only if the bomb actually started from Malta, which is a finding the defence have strongly challenged in the appeal," Prof. Black said.

The hearing continues despite rumours that the 57-year-old former Libyan intelligence officer may choose to drop his appeal and go home because of a recent prisoner transfer agreement between the UK and Libya.

Mr Al-Megrahi is suffering from prostate cancer and can choose to die at home. But dropping his appeal will leave him a condemned man and mean that Malta will remain implicated in one of the worst terrorist acts in aviation history.

According to Dr Koechler, it is "absolutely essential" that the appeal goes ahead: "The Scottish authorities can reconcile the imperatives of the rule of law and of humanity and grant the appellant compassionate release while the appeal goes on... In a situation where there are serious doubts whether he is guilty as charged, and where the public is confronted with an increasing number of shocking revelations about the mishandling of the case by the judiciary, tampering with evidence, and so on, it is appropriate to make such a step."

Dr Koechler believes the British Parliament should mandate an independent public investigation into the Lockerbie case.

"The international public, including the people of Malta, deserve to know the truth - the full and uncensored truth - about the chain of events that led to the explosion of the American jetliner over Lockerbie."