MORTON v HM Advocate is one of those cases that every law student in Scotland has studied for almost a century. For 87 years, the case has structured how corroboration has been understood and applied by every layer of our criminal justice system.

In their essays, generations of undergrads have dutifully trotted out Lord Justice Clerk Aitchison’s judgment from the case that corroboration “has proved an ­invaluable safeguard in the practice of our criminal courts against unjust conviction, and it is a rule from which the courts ought not to sanction any departure”.

Generations of defence lawyers have tested the evidence led against their ­clients against Morton’s thresholds.

Generations of police officers and procurators fiscal have marked criminal cases “no ­proceedings” – all because there was no corroboration in the terms that Morton demanded.

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Generations of judges and sheriffs have ­either thrown out prosecutions – or quashed convictions on appeal – when the available evidence fell short of this Morton standard.

And last week, the Lord Advocate asked a full bench of the High Court of Justiciary – a full nine judges – to overrule the better part of a century of legal practice.

Dorothy Bain KC argues the 1937 decision represented a “wrong turning” in Scots law.

She’s asking Lord Carloway and his colleagues to take us back to the future by endorsing a looser idea of what kind of corroboration is needed for criminal cases to be put in front of a judge or jury, particularly sexual cases.

If she’s successful, it will be one of the most radical changes Scots law has seen in decades, all without a single politician ­being consulted or a single vote being cast.

In essence, the law gives the chief ­public prosecutor the opportunity to ask the High Court to clarify a contested area of ­criminal law.

In the academy, most of us thought these references were effectively a dead ­letter – useful before devolution, with Westminster largely neglecting Scots criminal law – but superseded once a democratic parliament was re-installed in Edinburgh with a permanent interest in the justice ­system.

Inside the Scottish Parliament building

Bain’s tenure has challenged this assumption – reflecting her evident personal ­commitment to addressing how the criminal justice system responds to allegations of sexual crime.

That manifests too in the consistent appeals she’s lodged against “unduly ­lenient” sentences in rape cases.

Although largely unnoticed by the Scottish media – because it has been largely untrumpeted and unexplained by the Crown itself – a significant number of disposals have been referred back to the Appeal Court, ­resulting in some significantly enhanced sentences where “judge’s errors caused them to under-estimate the seriousness of the offence”.

Last week’s ­corroboration gambit is part of the Lord Advocate’s wider strategy.

First, the Scots law for Numpties ­version. The basic requirements of ­corroboration are fairly straightforward. Prosecutors need two independent pieces of evidence to prove the essential facts of any criminal case.

The first essential fact is that the crime was committed. The ­second is that it was the accused who committed it. If an allegation isn’t ­corroborated – then there’s insufficient evidence in law to convict. You might have corroboration for the first essential fact, but not the second. It doesn’t matter. The case fails.

In essence, this means that nobody can be convicted of a crime in Scotland on the evidence of a single witness, ­however credible or honest they may seem.

In ­contrast with the rest of the UK, in ­Scotland there always needs to be ­something extra – something ­independent of the ­complainer’s testimony – for a ­conviction.

This is part of the reasons why the ­police send officers on the beat in pairs. In Scots law, two police officers can ­mutually ­corroborate one another’s ­evidence that the unfortunate citizen they’ve huckled had committed a breach of the peace.

The evidence of a single officer wouldn’t do.

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Corroboration doesn’t always require a second witness – an impossibility in many cases where domestic abuse or ­sexual ­offending takes place behind closed doors.

Circumstantial evidence is used all the time in our courts – and will count for corroboration if it ­“strengthens, ­confirms or supports” the evidence a ­witness has given.

That all might sound a little abstract. The facts in Morton demonstrate what’s at stake in practice.

In November 1936, in pre-war Glasgow, a young woman was hustled into a tenement close in ­Annbank Street in Dennistoun. The man was a stranger to her. He attacked her. She fought back, cried out – and made to ­escape.

The High Court in Glasgow  (Image: Newsquest)

This commotion drew the ­attention of a neighbour living in the flats opposite. Peering out her window, she saw what was happening – and started shouting.

Startled and interrupted, the ­attacker made his bid for freedom and ran off. Critically – this neighbour didn’t see his face.

The victim returned home, shaken, and told her brother what happened. She ­complained to the police. Her brother ­corroborated her account of events, ­confirming she seemed distressed and upset and confirming she told him she’d been attacked by the stranger. But ­obviously – he didn’t see the attack or the attacker himself.

He’s backing up her story, but only in the aftermath.

Two months later, in January 1937, the victim was able to pick Henry ­Morton out of a police lineup as the man ­responsible. There were no other witnesses to what happened.

Arrested, charged and ­prosecuted – the jury convicted Henry Morton and the Sheriff handed him 12 months’ imprisonment. His lawyers ­appealed. Their argument? Where’s the corroboration here?

Work the logic through.

While the ­victim and the witness on Annbank Street could both confirm the victim was assaulted – the court only had the ­complainer’s direct evidence that Henry Morton was the man who attacked her. The independent witness couldn’t say Morton looked like the man she’d seen. Her brother wasn’t there.

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In 1937, the High Court quashed Henry Morton’s conviction. While there was sufficient evidence in law that somebody had attacked this woman, they held there was only a single strand of evidence proving that Morton was that attacker – one strand too few.

The fact she confided what happened to her family when she got home was just a repetition of her own evidence. It was certainly relevant to her credibility and reliability but it couldn’t provide any truly independent evidence to support the fact that she was attacked or that Morton was her attacker.

This really is the nub of last week’s Lord Advocate’s reference.

Bain argues this conclusion wrong – and that the brother’s testimony should be capable of corroborating not only the attack but also the identification of the attacker. It is this second argument of these arguments which is the most radical.

Many lawyers believe the outcome of this case is already a foregone conclusion. The two references were launched this week just as Lord Carloway announced retirement as Lord President next year.

He will retire having spent 25 years on the bench, ten of them as the country’s most senior judge.

Thirteen years ago, he recommended in a report for the Scottish Government that corroboration should be abolished.

His argument? The doctrine had become unduly technical, difficult for judges to explain, and difficult for ­juries to understand. Under Carloway’s leadership in the years since, there is a widespread ­perception in legal circles that the High Court has used a succession of cases to effectively hollow out the corroboration doctrine, dismantling its historic requirements, overruling technical caselaw, and lowering the standards of corroboration required.

Critics of corroboration broadly agree with this analysis but have welcomed these changes, arguing that it stops cases being excluded from court for ­technical reasons, allowing juries to decide ­whether they’re satisfied ­whether the Crown has proved its case beyond reasonable doubt.

For Lord Carloway, overruling Morton v HM Advocate may be a final judicial mic-drop.