IN the case for reform of Scotland's criminal law, there can be few more impressive witnesses than the chief public prosecutor.

The Lord Advocate, Frank Mulholland, tells The Herald today that the law is no longer serving the victims of sexual offences as well as it should. He says the corroboration rule, combined with the impact of the Cadder ruling, mean there is now an imbalance in favour of the accused, particularly in rape cases.

For anyone who values the idea that Scotland's criminal justice system should be balanced and fair, Mr Mulholland's comments are worrying, but they are also the latest indication that, even though the rule of corroboration has long been seen as one of the fundamentals of Scots law, the status quo is no longer acceptable. Where we go from here is less certain.

At least as far as the Cadder judgement is concerned, there no going back. In 2010, the Supreme Court ruled that allowing a rape accused to be interviewed by the police without a solicitor, as was customary in Scotland, was a breach of the European Convention on Human Rights. Mr Mulholland says this ruling has put the criminal justice system out of balance, but it must be right that an accused has a lawyer present. The European Convention says this is so and the convention is part of Scots law.

More worrying is the effect the Lord Advocate says Cadder is having on convictions. Pre-Cadder, a suspect would often admit to the police that sex took place but deny rape and this statement was then often used as corroboration in a later trial. Post-Cadder, with lawyers present at the police interviews, suspects are more likely to say nothing which means it is harder to meet the corroboration test. Mr Mulholland says this means many cases in which there is a good chance of the complainer being believed are now failing because the prosecutors do not have the corroboration required.

At a time when rape conviction rates in Scotland remain shockingly low, this is worrying. Last year, data from the Crown Office and procurator fiscal service found 67% of sexual crimes that failed for lack of corroboration would have had a reasonable prospect of conviction without the requirement for corroboration. The natural response is to suggest, as Mr Mulholland has, that the requirement be removed.

Victims' group have supported this idea and it is easy to see why. Corroboration has often been seen as the main obstacle to conviction. But there is no settled view on this among campaigners and lawyers. The Scottish Human Rights Commission and the Law Society of Scotland have both defended corroboration. The president of the Law Society, Cameron Ritchie, has also said the corroboration rule should only be removed as part of wider reform; interviewed in The Herald last week, Gillian Wade, the head of the National Sexual Crimes Unit, expressed a similar view.

Even so, the case for removing the need for corroboration is becoming more compelling. There may be some who are worried the reform would achieve more convictions at the expense of fairness to the accused, but this should not necessarily be the case. Even after changing the rules, there would still be strict rules on the evidence needed, not least that criminal cases should be proved beyond reasonable doubt. There is also nothing to suggest that removing corroboration would make a jury any more or less likely to believe a witness – a jury would, as always, reach a verdict on the evidence and not necessarily on the basis of a strict corroboration rule.

The case for removing corroboration may not be proved beyond reasonable doubt yet, but it is a case that is becoming increasingly hard to resist.