A WITNESS who provided the crucial evidence 10 years ago that convicted a former policeman of the murder of teenager Lorna Porter is a ''fantasist'' who now admits that he made up his testimony, the Court of Criminal Appeal heard yesterday.

George McAvoy was in the habit of phoning the police in the early morning under the influence of drink to make serious allegations of criminal conduct against people, the court was told.

Former Royal Marine Alexander Hall was found guilty at the High Court in Glasgow in February 1988 of attacking the 18-year-old victim in the garden of his home in Thorn Road, Bellshill, and cutting her throat.

The murder took place in September 1984, but it was more than three years before Hall, who had resigned from the police force shortly before the killing, stood trial.

He was alleged to have made a confession to police, and the Crown also relied heavily on the evidence of George McAvoy, who claimed that at about the time of the murder he had seen an argument between a man resembling Hall and a woman whose description fitted Miss Porter.

Hall's appeal after conviction was turned down.

Yesterday, Mr Michael McSherry, solicitor-advocate for Hall, argued to Lord Cullen, the Lord Justice-Clerk, who is hearing the appeal with Lords Kirkwood and Osborne, that there was now significant additional evidence to cast serious doubt on the credibility and reliability of George McAvoy.

Lorna Porter had been engaged to Hall's brother-in-law. After the murder Hall had been interviewed several times and his home searched. There were various pieces of circumstantial evidence, but even when added together these were of minor importance, said Mr McSherry.

Then, in July 1985, Hall had been arrested by police for road traffic matters and, allegedly in a highly emotional state, made what police claimed was a clear confession to Lorna Porter's murder. He was alleged to have said: ''I've got something terrible to tell you. I did Lorna Porter. I've nothing else to live for. I'm going to kill myself.''

Hall denied that he had confessed to the murder. Mr McSherry said that even after the alleged confession the Crown did not feel justified in prosecuting Hall. The crucial development was when George McAvoy came forward after reading a newspaper article about the unsolved crime.

He claimed had seen a woman in a white coat or jacket in Hall's garden arguing with a man with a beard and collar length hair.

Mr McAvoy claimed to have seen the man grab the girl and bring her arm up behind her.

That was in August 1987. Hall had been arrested in October that year and committed for trial.

Mr McSherry told the court: ''It is being alleged by the defence in this case that when he phoned up the police in 1987 and told them he had seen something happening in the garden he was fantasising.''

Mr McAvoy had been interviewed by police in 1995 about his evidence at the trial and had admitted that what he had told the jury about seeing people in the garden was untruthful.

He had been asked why he had come up with the story at the time and replied: ''Don't know. Just made it up.''

Mr McSherry said: ''He admits that he made up the allegation that he had seen the man and woman in the garden of Hall's home. The evidence of George McAvoy goes right to the heart of the decision of the jury which I understand was by a majority.''

Hall also wants to lead evidence of statements by Mr McAvoy's brother, James, which state that George McAvoy could not have been in a position to see and hear what he alleged was taking place in the garden in Bellshill.

This evidence was not available at the trial.

The hearing continues.