The future of the Scottish legal system's unique not proven verdict will be debated for the first time by the Holyrood parliament after a back bencher launched a campaign for its abolishment yesterday.

Michael McMahon has published a public consultation document on the matter which could lead to major changes to the criminal law.

The verdict, famously dubbed "that bastard verdict" by Sir Walter Scott, has been the subject of intense debate among the Scottish legal establishment over the past two centuries.

The essence of a not proven verdict is that it results in an acquittal, with the same legal consequences as a not guilty verdict, entitling the accused to walk free from court and not to be tried again for the same crime.

Criticism of the verdict, which is unique to the criminal law of Scotland, stems from a sense of injustice most often expressed by the families of murder victims.

Mr McMahon, the Labour MSP for Hamilton North and Bellshill, believes juries should have only two, rather than three, verdicts to choose from and that these should be either proven and not proven, his preferred combination, or guilty and not guilty. This would, according to Mr McMahon, sweep away confusion and misunderstanding and reinstate some balance to the criminal justice system.

He claimed yesterday that certain lawyers wanted the not proven verdict retained because they could "go back to the bar and congratulate each other on getting someone off on the not proven verdict."

He said it was the public that had been calling for a change to the system and it was they, rather than certain lawyers, who should be listened to. The MSP said some legal figures were more interested in "serving their own interests" than those of the public, and thought they had a "clever system" in place.

This will be the first time that the Scottish Parliament has considered the issue of the not proven verdict and MSP Mr McMahon hopes his proposal, which will come under the Criminal Procedure (Reform of Verdicts) (Scotland) Bill, will be given a smooth path. He has asked for public comments on his proposal which have to reach him by June 19.

Because the not proven verdict would be removed, he has suggested that the majority required to reach a decision on a defendant should be changed. At present, a simple majority of one is required to return a guilty verdict.

Mr McMahon said requiring a larger majority, be it two-thirds or three-quarters, would provide a better balance where the third verdict was not available.

"I think the not proven verdict is something that, after eight years, the Scottish Parliament should have a debate about," he said. "I know it is a contentious issue but it is time for us to consider it.

"The distinct Scottish judicial system is something of which the country is rightly proud but the anomaly of having not proven, which essentially means there are two possible acquittals, brings it into disrepute.

"It's something that is illogical, anachronistic and just plain wrong. There have been too many occasions on which victims or their families have been left aggrieved at a not proven outcome. Equally there are times when accused persons have been left with a cloud of suspicion hanging over them."

A spokesman for the Faculty of Advocates said: "Although there may be concern about the existence of the not proven verdict, this should not be viewed in isolation but against the background of other elements of the Scottish criminal justice system.

"The ability of a jury to have available a third verdict may be important where a verdict of guilty can be delivered by a simple majority of 15 jurors and in a system where there are no pre-trial committal proceedings before an accused is bought to court such as exist in other jurisdictions."

An issue for 200 years

The debate over the not proven verdict has been going on for over 200 years: Sir Walter Scott famously dubbed this acquittal decision as "that bastard verdict", such was the level of outrage he noted at the end of a particular court case.

Over the past three decades there have been three reviews of the Scottish verdict system. None of the reviews decided to scrap the not proven verdict.

Westminster debated the not proven verdict in 1969, when former First Minister, Donald Dewar, introduced a Ten Minute Rule Bill to abolish it, and again in 1993 when George Robertson tabled a private member's bill. Both failed.

First Minister Jack McConnell and former Lord Advocate Colin Boyd were both consulted before Mr McMahon put his consultative paper on the not proven verdict together.

Figures show that in 2004/05, 18% of all people acquitted were found not proven, figures which Mr McMahon said showed the verdict was still being used.