Appeal judges were told today there "no positive identification" of a Libyan intelligence officer by a crucial witness at the Lockerbie bombing trial.

A senior counsel said there were "striking discrepancies" in the evidence of a Maltese shopkeeper over the height and age of a man who had bought clothing from him with that of Abdelbaset Al Megrahi.

The clothes were found to have been in a suitcase which housed the bomb that blew Pan Am Flight 103 out of the skies over the Dumfriesshire town in December 1988 killing 270 people.

Margaret Scott QC told the Court of Criminal Appeal in Edinburgh that the testimony of shopkeeper Tony Gauci was at best "a looks like resemblance" between the man who made the purchases and Megrahi.

She said: "When one looks at the identification evidence it is incapable of sustaining a finding that the appellant was the purchaser of the clothing."

The finding was one of four critical inferences made by judges at Megrahi's original at Camp Zeist in the Netherlands in convicting him of murder in 2001.

Megrahi (57) whose health is "deteriorating" as he suffers from prostate cancer, is appealling against the conviction claiming he was the victim of a miscarriage of justice.

He was jailed for life following the guilty finding and ordered to serve at least 27 years for the mass murder.

Megrahi has previous unsuccessfully challenged his conviction, but his case has now been referred back to the appeal court by the Scottish Criminal Cases Review Commission, which examines alleged miscarriages of justice.

His counsel, Miss Scott, said that in 1989 Mr Gauci had described the man who bought the clothes as aged about 50 and six feet in height. Megrahi was aged 36 at the time of the purchase and stood five feet eight inches tall.

"The initial description given by the witness at the outset is substantially different to the appellant both in terms of height and age," she said.

She said Mr Gauci had been shown several photospreads by police on different occasions as they sought his help.

Miss Scott said that at the first which featured a photo of Megrahi, supplied by the FBI, there were aspects of procedure clearly different to the others.

She said initially Mr Gauci said the men featured were younger than the purchaser.

The defence counsel said: "In a sense he rejected the photos on the basis they were too young, but quite unlike before the witness was told to look at the photos again carefully and to try to allow for any age difference."

Miss Scott argued it was "a clear message that the witness needs to try again and a message that there is something there to be found".

She said it was only following this that Mr Gauci picked out the photo of Megrahi as being similar to the man who bought the clothing.

"In my submission, that is highly irregular and liable to introduce the risk of significant error in what he subsequently does," she said.

Miss Scott said that an identity parade held at Camp Zeist in 1999 with Mr Gauci in attendance was also flawed.

She said no other Libyans were part of the line-up and four of the participants were in their 30s and one was five feet three inches tall. "Four people were quite unreasonably young and one was unreasonably short," she told the court.

Mr Gauci picked out Megrahi at the parade as a man "who look a little bit like exactly" the clothes buyer.

The defence counsel said: "It is quite clear there has been no positive identification of the appellant as the purchaser. At best the witness makes a form of resemblance identification."

The hearing before five judges continues.