The Libyan secret service agent serving life for the Lockerbie bombing has been given until December 21 to explain his reasons for challenging his conviction.

The date is exactly 19 years since the blast which killed 270 people when a US-bound aircraft exploded in a ball of fire above the Scottish town.

At the Court of Criminal Appeal in Edinburgh today Jim Swire, whose daughter, Flora, was among those who died described it as "a very ominous date."

"We are getting near the nineteenth anniversary of the murder of out loved ones," he said.

Mr Swire also said he was pleased that Scotland's top judge, Lord Hamilton, seemed to want to speed things up as much as possible but, on the other hand, it would be wrong to put too much pressure on the defence.

Mr Swire was among those in court to hear that the legal team fighting for the release of Abdelbaset al-Megrahi, 55, demand the handover of secret documents which they believe could prove his innocence.

Defence QC Margaret Scott also claimed that a new witness could discredit Maltese shop keeper Tony Gauci whose evidence was crucial in convicting Megrahi at a special court in the Netherlands in 2001.

Ms Scott also said defence forensic experts were working on reports to counter other evidence led at the trial.

The hour-long hearing in Edinburgh followed recent speculation that United States security services were blocking the handover of potentially crucial information about the timer which detonated the bomb on PanAm flight 103.

But today, Scotland's top judge Lord Justice General Lord Hamilton, sitting with Lords Kingarth and Eassie, heard that the Americans were not involved.

"The documents don't come from that government or any of its agencies," said advocate depute Ronald Clancy QC, for the Crown.

He told the court: "The documents in question were passed to the UK Government on the basis that they were regarded as being confidential by the authorities that passed them over.

"That being so, the Crown has always taken the position that, if possible, confidentiality should always be respected."

Mr Clancy added: "The Crown has been actively pursuing the matter but today it remains unresolved."

Requests had been made to allow the Crown to hand over the documents and it was possible this might happen without the appeal judges having to rule on the issue, the court heard.

Mr Swire said if the secret documents did not come from the United States it was "pure speculation" which government they belonged to.

The bombing of the United States-bound Pan Am flight in 1988 is regarded as the worst crime in Scottish legal history because of the death toll among passengers and on the ground.

After much diplomatic wrangling al-Megrahi and another Libyan, Al-Amin Khalifa Fhimah surrendered and were put on trial in a special high security courtroom constructed at the Zeist military installation near Utrecht.

Three judges heard the evidence without a jury and aquitted Fhimah.

Five judges then heard an appeal in 2002 against conviction by al-Megrahi but decided that the guilty verdict for the murder of 270 people should stand.

For a while al-Megrahi was held in a special cell in Glasgow's Barlinnie jail - dubbed the "Ghadaffi Cafe" - but is now in Greenock Prison, serving a minimum of 27 years.

The Scottish Criminal Cases Review Commission, which investigates possible miscarriages of justice, took up the case and after a lengthy investigation concluded in June that al-Megrahi should get a second appeal.

It was during the SCCRC probe that the existence of documents shown to prosecutors but unseen by the defence emerged.

"The Commission then reached the conclusion that the Crown position not to disclose at least one of these documents to the defence indicated a miscarriage of justice might have occurred in this case," said Ms Scott today.

She told the appeal judges that without all the material she could not formally lodge detailed grounds of appeal and asked for more time to prepare.

Lord Hamilton granted the request and made December 21 the deadline.

The court heard that, in outline, al-Megrahi's lawyers will argue that there was not sufficient evidence in law to convict him and that a jury would not have found him guilty.

There are issues about the conclusions of forensic experts and whether the defence lawyers at the original trial did a good job and identification evidence.

Shop-keeper Mr Gauci told the Kamp Zeist trial that al-Megrahi was the man who bought clothes and an umbrella said to have been in the suitcase which also contained the bomb, disguised as a radio cassette player.

Ms Scott also told appeal judges that unless they could guarantee a safe escape route for al-Megrahi in the event of a successful appeal, the full hearing - which is expected to take place some time next year - would have to be in the Netherlands.

Al-Megrahi was not in court for today's hearing.