The conviction of Sally Clark, sentenced to life in November 1999 for the murders of her two babies, Christopher and Harry, has recently been referred back to the Court of Appeal by the Criminal Cases Review Commission (CCRC). The appeal will be heard on the basis of newly disclosed medical reports from two pathologists which state that the second child died of an infection (there is already evidence that the first died from natural causes).

If correct, it will mean not merely that Clark has been wrongly convicted, but that she has served nearly three years in prison for crimes that never happened, that this was not a dramatic and high-profile murder in which there was public pressure on the authorities to secure a conviction, but an illusory crime.

The Clark case seems one among many. Given the increasing tendency to find culprits for tragedies, a significant number of serious cases are reaching court in which no crime may ever have been committed.

In April Angela Cannings was convicted of the murders of two of her children. The defence maintained that there was nothing suspicious about either death and brought forward 12 distinguished scientists who gave supporting evidence. The prosecution had its own medical evidence. With high- level medical views so divided, was it sufficiently clear that these crimes had taken place?

Maxine Leggatt was similarly convicted after her two young children died in bed on the hottest day of the year in 1993. There was plenty of evidence to suggest that Maxine was a doting mother; and no reliable medical evidence that either child was murdered. Yet she, too, is serving life imprisonment.

Driving home in 1995, Squadron Leader Nicholas Tucker swerved to avoid deer on the road. The car went down a bank and into a small river; his wife drowned. The tragedy was compounded by a lengthy police investigation and Tucker's conviction for murder. The body had been examined by two of the most respected pathologists in the country, Professor Bernard Knight and the late Iain West, neither of whom could discern any evidence of murder. Tucker is serving life.

Similarly, Jon Probyn's wife died when her car crashed into the River Severn late at night in September 1992. She was estranged from her husband, they had had a blazing row that evening and were due in court in Gloucester the next day to dispute custody of their children. Suspicious circumstances? The police thought so. They construed the accident as murder and Probyn is serving life. The defence contended that this had been a simple, albeit tragic, road accident.

Jong Rhee is serving life for the murder of his wife, Natalie, who was killed in a fire at a guesthouse in the Snowdonia National Park. His defence is straightforward: "There was a fire, I escaped, but sadly my wife didn't." Forensic experts for the defence said there was no evidence of foul play.

Then there are the sexual abuse cases, ranging from "institutional" cases, in which those convicted argue that complaints have been artificially generated by police trawling inquiries; to "domestic" cases, in many of which prisoners maintain that a bitter family breakdown has led to malicious, and unfounded, accusations. There are hundreds of prisoners in this position.

These cases of murder or abuse, many within the family, are difficult to defend and acquittals are rare. In one "abuse" case, the defendant could prove that he was not in the country when the crimes were alleged to have taken place, yet he too was convicted. Many of these cases are with the CCRC.

In its original assessment of whether a case should be taken to trial, the Crown Prosecution Service needs to satisfy whether the prosecution is in the public interest and whether there is a greater than 50 per cent chance of a conviction. Yet, with these cases, the "public interest" is no test at all, since the charges are so serious that it is inevitably met. The other test is also easily satisfied. I believe a third criterion should be introduced: whether it is sufficiently clear that a crime has occurred. This would provide a vital safeguard, preventing many of these contentious cases from reaching court at all. After all, if the authorities are wrong about only some of these "convictions without a crime", then it means the Exchequer is suffering a financial loss that is not only staggering, but also wholly avoidable.