When should you be allowed to bring a private prosecution? The very idea that a private individual may be able to initiate the state’s powers to prosecute and punish offenders may strike some people as strange. Isn’t that the job of public prosecutors, such as the Crown Prosecution Service (CPS)?

Indeed it is, but the right of individuals to institute criminal proceedings is of great antiquity, as the lawyers like to say. It was expressly preserved by the acts of parliament that created the director of public prosecutions (DPP) in 1879 and the CPS in 1985. Sir Theobald Mathew, DPP from 1944 to 1964, was fond of saying that his title was a misnomer because he directed nobody and there were no public prosecutions. He was right; police and public officials have no greater powers than anyone else to initiate prosecutions.

But there is a catch. For some 400 years or more the attorney general has been able to stop any criminal case by sending the court what is called a nolle prosequi. The power to take over a private prosecution has been enjoyed by the DPP since 1908 and by the CPS since it was set up. The courts have held that the power to take over a case includes the power to discontinue it.

But on what grounds? Sir Thomas Hetherington, the last DPP to serve before the CPS was set up, made it clear in 1978 that he would take over and shut down a private prosecution only in very exceptional cases. In his view, protecting defendants against an unjust prosecution was the job of the courts. When the CPS started work in 1986, its policy was to stop private prosecutions only if they represented a genuine injustice to the defendant. By 1998, two other factors had been added: a case would be stopped if ‘the public interest factors tending against prosecution clearly outweigh those factors tending in favour’ or there was ‘clearly no case to answer’.

But ‘no case to answer’ is a higher threshold than the test in the CPS’s own code: that a case should go to court only if there is a ‘realistic prospect of conviction’. It followed that there would be some cases where the CPS would permit a private prosecution even though the evidence would have been insufficient to justify a public prosecution.

That approach led to some high-profile cases in which the CPS allowed private prosecutions even though it had decided against bringing charges itself. Inevitably, some failed. In 1994, for example, the family of the murdered black teenager Stephen Lawrence prosecuted five white youths. Charges against two defendants were later dropped for lack of evidence. The remaining three were acquitted by a jury after the judge had found that the only available evidence was unreliable. If the law on double jeopardy had not been changed in the meantime, that acquittal would have prevented one of the three, Gary Dobson, from being convicted of the murder at the beginning of this year after fresh evidence had come to light.

Another private prosecution arose from the Hillsborough disaster of 1989, in which 96 football fans were killed. The DPP decided the following year that there was no evidence to justify criminal proceedings against anyone associated with the tragedy. In 1999, however, the victims’ families launched private prosecutions for manslaughter and misconduct in public office against two former police officers who had been in overall command at the match. At the trial in 2000, superintendent Bernard Murray was acquitted and the jury could not reach a verdict on chief superintendent David Duckenfield. An application for a retrial was refused.

Before the case had come to court, the two defendants had tried unsuccessfully to persuade the CPS to step in and stop it. They then challenged the CPS refusal by applying for judicial review.

Dismissing the application in 1999, Lord Justice Laws took the view that the CPS was under no duty to take over and stop a case that did not have a realistic prospect of conviction; indeed, adopting such a policy would have been unlawful. But in 2009 the CPS did just that, announcing that it would now stop private prosecutions that did not meet the evidential sufficiency test of the full code.

Last year, a court headed by Lord Justice Richards managed to find that Laws’s reasoning was obiter, meaning that it did not have to be followed and the new CPS policy was lawful. Richards dismissed a challenge by Dalvinder Singh Gujra, who wanted to bring common assault charges against some people he knew. Gujra appealed to the Supreme Court last week, though his chances of success do not look good.

Other private prosecutors risk having to pay hefty costs, at the very least. Babar Ahmad and Syed Ahsan were among five men who were extradited to the US last Friday on terrorism charges. Two days earlier, the chief magistrate had refused to allow Karl Watkin, a businessman who campaigns against US-UK extradition, to launch a private prosecution against them for the offence of soliciting to murder. The magistrate found that the application – designed to delay the extradition – was an abuse of the process of the court, a conclusion endorsed by the High Court on Friday.

Private prosecutions perform a useful function, allowing cases to be brought by institutions ranging from department stores to the RSPCA. But it would be oppressive if they could be used to put people in the dock who have no realistic prospect of being found guilty.