A previously-unseen witness statement is expected to undermine the identification of the man convicted of the Lockerbie bombing, the long-awaited appeal which begins today will unveil.

The appeal of Abdelbaset Ali Mohmed al Megrahi, the man serving 27 years in prison for the bombing, will start with an overview of the different arguments, including the Crown's failure to disclose certain key statements.

These will include the previously unseen statement of David Wright, a friend of Tony Gauci, the Maltese shop owner whose identification of Megrahi was crucial to the conviction.

Mr Wright allegedly gave a "remarkably" similar description of a sale made at Mr Gauci's shop in Malta to the one used to implicate Megrahi. He gave a statement to English officers in December 1989.

A source said: "The new witness provides an account which is startling in its consistency with Mr Gauci's account of the purchase but adds considerable doubt both to the date of the purchase and the identification by Mr Gauci of Megrahi as the purchaser."

Details of his statement, and whether it contradicted Mr Gauci's evidence, were not presented at the trial and had not been seen previously by the defence team.

Mr Gauci claimed that he had sold the former Libyan intelligence agent clothes which were inside the bomb-carrying suitcase which brought down Pan Am flight 103 over Lockerbie, killing all 270 people.

The hearing before the Scottish Court of Criminal Appeal in Edinburgh, is due to sit for four weeks at a time with a month's break in between.

The defence team will question why the original trial excluded the incrimination of a terrorist cell that was operating in Germany shortly before the tragedy and why an inconsistent witness paid financial reward, could have been credible.

They will raise concerns about the trial's exclusion of the defence case to incriminate Abu Talb, who was subsequently convicted in Sweden of terrorist offences, and other members of the Popular Front for the Liberation of Palestine - General Command (PFLPG-C), the terrorist cell that was operating in Germany before the Lockerbie bombing.

They will argue that his right to a fair trial has been breached and that the original case was not proven.

The appeal will also scrutinise the trial court's finding that the suitcase carrying the bomb was put on the plane at Luqa airport in Malta.

The case was referred back to the appeal court in June 2007, following a long investigation by the Scottish Criminal Case Review Commission which concluded it may have been a miscarriage of justice on six separate points.

However, Megrahi, who is suffering from terminal prostate cancer, could be transferred home to Libya under an agreement being rushed through parliament by Jack Straw, the UK Justice Secretary. While he is keen to clear his name in court, there is concern that he may not survive the long appeal process.