The UN observer at the Lockerbie trial, Hans Köchler, has said that the Libyan convicted of the bombing will not get a fair hearing in Scotland.

Köchler, who advises the European Commission on democracy and human rights, has condemned government interference in the appeal of Abdelbaset Ali Mohmed al-Megrahi and said the hearing should be held in a neutral country.

His intervention follows an attempt by the British government to block the release of secret papers that could help clear the former Libyan intelligence agent convicted of the 1988 bombing, which claimed 270 lives.

Köchler said Megrahi’s case was handled “more like an intelligence operation than a genuine undertaking of criminal justice” and criticised MSPs for failing to hold inquiries into the downing of Pan Am 103 and its judicial aftermath. “It is almost trivial to say that a fair trial requires the availability of evidence to the prosecution and defence. Only in a totalitarian system would the executive power interfere in court proceedings and order the withholding of evidence.”

The Advocate General, on behalf of British ministers, had objected to disclosure of the documents to Megrahi’s legal team, lodging a public interest immunity plea.

Last month senior judges ordered that the papers should be released to the Court of Criminal Appeal in Edinburgh, where a panel of three judges will decide in camera whether they should be disclosed.

The documents, which are believed to hold information about the electronic timer that detonated the bomb, were not disclosed to the defence during al-Megrahi’s trial at Camp Zeist in the Netherlands. Megrahi lost an appeal in 2002, but the Scottish Criminal Cases Review Commission concluded that he might have been the victim of a miscarriage of justice and referred his case back to the court last year. One of the grounds for referral is believed to be the prosecution’s failure to disclose the secret document to Megrahi’s lawyers.

Köchler said the decision to hear the appeal in Scotland breached a concordat between the UK, the US and the Netherlands. “The fact that the new appeal proceedings take place in Scotland is not in conformity with the original intergovernmental agreement on the Lockerbie trial.” The proceedings totally lacked “transparency”, he said.

Last week, Robert Black, the Edinburgh law professor who helped to arrange Megrahi’s original trial in the Netherlands said the intergovernmental agreement no longer applied. It “existed for the original trial and the appeal. This is now the second appeal.” The agreement was spent, he said.

“Scotland made a mess of the trial and the appeal, and to an outside observer, that might lend justification to Köchler’s view. But I believe that this time it will be done properly and Megrahi will be released.”

Last year, Köchler said Scotland had the reputation of a “banana republic” because of its handling of the case.

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