The legal challenge, at a hearing in the Netherlands, follows secret visits by senior Scottish prosecutors in the case to interview a key prosecution witness in America, who has been in hiding for the last eight years. A team led by the prosecutor Norman McFadyen last month met the Libyan witness who claims to have seen the two defendants plan and construct the bomb, which killed 270 passengers and people on the ground.
However, at tomorrow's hearing at Camp Zeist, a former American air base, lawyers for the two defendants will argue that the Scottish judges who will hear the case have no jurisdiction to try the conspiracy charge. Success by the defendants on this front would reduce the significance of the testimony from the prosecution's star witness. Defence lawyers will say that since the alleged conspiracy took place outside Scotland, the charge should not be presented to a Scottish court.
Abdel Basset Ali al-Megrahi, 47, and al-Amin Khalifah Fhimah, 43, deny charges of conspiracy to murder, murder and contravention of the Aviation Security Act of 1982. If there is any reasonable doubt as to their guilt, then the three Scottish judges who will try the case must find them either not guilty or the charges not proven.
The credibility of the key witness will be crucial to the prosecution's case, when it is eventually presented at the full trial in Camp Zeist, which begins on February 2. The defence will aim to undermine his credibility by showing that he was either misled or is not telling the truth. But the main focus of their case will be that someone else - a Middle Eastern group - was directly responsible for the destruction of the aircraft.
The man's identity is known to The Independent but has been a closely guarded secret until now. He has been in a US witness protection programme in undisclosed locations in America since at least 1992. He claims to know the two Libyan defendants, who were officers in the Libyan security services and who worked at the Libyan Arab Airlines (LAA) office in Malta, from which the prosecution alleges the bombing was masterminded. Mr Fhimah was working under cover as station manager for LAA in Malta; Mr Megrahi was chief of the security service's airline security division.
When the aircraft blew up, the man who is now the key prosecution witness is said to have been terrified and to have defected. He is believed to have walked into the US Embassy in Rome as a defector.
The indictment against the alleged bombers - who were first named three years after the explosion - depends heavily on this man's testimony.
Mr McFadyen took a pre-trial statement from the witness, which will be used in the case against the two Libyan defendants. The witness has been in almost total seclusion for at least eight years, fuelling speculation that he may be in a delicate mental state.
The Crown Office in Edinburgh, which represents the prosecution, has declined to comment on its contacts with the witness. The defence team will also be allowed to meet the witness, though it is uncertain whether or not this has yet taken place.
Under Scottish law, there should be at least one other witness, to corroborate his testimony. About one third of the people on the 1,000-strong witness list come from the US, and many, like the key witness, have their addresses as the US Justice Department in Washington.
Most are FBI Agents but there are thought to be others on the witness protection programme. Like the Libyan witness, they are expected to give their testimony to the trial in the Netherlands from behind a screen.
The defence will centre its efforts on the Scottish special defence of incrimination: that another group of people, not their clients, were responsible. They will blame individuals linked to Ahmed Jibril's Popular Front for the Liberation of Palestine - General Command, a Palestinian splinter group with ties to Syria. It is believed the motive for the bombing could be revenge for the US shooting down an Iranian airliner over the Gulf in July 1988.
EVENTS LEADING UP TO THE TRIAL
21 Dec 1988: Pan Am Flight 103 blows up over Lockerbie; 270 people killed including 193 US citizens.
14 Nov 1991: Britain and US accuse two Libyans, Abdel Basset Ali al-Megrahi and al-Amin Khalifah Fhimah, of the bombing. Libya denies involvement.
21 Jan 1992: UN orders Libya to surrender suspects for trial in Scotland or the United States.
15 April 1992: UN air and arms embargo on Libya.
2 Oct 1993: Britain, US and France tighten sanctions.
27 May 1995: John Major rejects Nelson Mandela's (above, left) call for trial in a neutral country.
11 Dec 1996: Abdel Basset Ali al-Megrahi agrees to be tried in a neutral country.
22 July 1998: Britain and US agree to hold trial at The Hague under Scottish law.
6 Dec 1998: Kofi Annan, the UN secretary general, visits Colonel Gaddafi (above, right), the Libyan leader, and asks him to hand over the suspects before the 10th anniversary of the bombing.
5 April 1999: Suspects extradited into Scottish custody and held at Camp Zeist, a former US base in the Netherlands.
29 Oct 1999: Indictments served to suspects charging them with conspiracy, murder and a breach of the 1982 Aviation Act.
7 Dec 1999: Abdel Basset Ali Mohmed al-Megrahi and al-Amin Khalifah Fhimah make their first appearance at a two-day pre-trial hearing at Camp Zeist before Lord Sutherland.
2 Feb 2000: Trial of the two defendants scheduled to begin before three Scottish judges at Camp Zeist.
BOTH SUSPECTS are believed to be connected to Libyan intelligence organisations.
Abdel Basset Ali al-Megrahi, 47, is allegedly a former director of Libya's Centre for Strategic Studies and a Libyan intelligence officer. Married with two children, he was born in Tripoli and speaks good English following his time as a student in the US. He has denied any involvement in the bombing, telling one reporter: "You judge me falsely. I'm a quiet men. My life is clean."
Mr Megrahi is said, in the formal charges, to have been head of security for Libyan Airlines at Luqa in Malta. He is accused of ordering 20 electronic timers from a Swiss company and testing them at a Libyan special forces training site. He also accused of ordering a further 40 timers after establishing a false travel company in Malta known as Med Tours as a cover for Libyan intelligence operations.
He is accused of putting a Samsonite suitcase on board a flight for Frankfurt at Malta's Luqa airport, which contained an umbrella, clothing and a radio cassette recorder. That bag eventually found its way onto Pan Am 103, which exploded over Lockerbie. After putting the baggage on the plane, it is alleged that he left Malta for Tripoli on a false passport.
Mr Megrahi is said to hold four Libyan passports and nine aliases. He and his children's passports were seized in 1991 when he was accused of the bombing. His home in Tripoli has been under armed guard since. He has relied since 1991 on a small pension from Libyan Arab Airlines and has worked as a teacher.
Meanwhile, al Amin Khalifa Fhimah, 43, is likewise believed to have been a Libyan intelligence officer, working undercover at Luqa airport, where he is claimed, under the conspiracy charge, of having been the station manager for Libyan Airlines. He is co-accused of placing the suitcase containing explosives on the plane bound from Malta to Frankfurt.
Married with five children, he has always insisted that his only interest was in tourism, having set up the Med Tours business with a Maltese partner and Mr Megrahi. He has told reporters that he is a peace-lover and was "neither an intelligence man nor a politician". Before his incarceration at Camp Zeist, he lived in Tripoli, and was born in Suk Giuma in Libya.
Although apparently the junior of the two accused, much of the evidence against both men is believed to come from his diary, which he said was stolen by Western intelligence officers. Like Mr Megrahi, he has had to live under armed guard since 1991 and his wife has had to work to support the family.Reuse content