The jury in the English legal system holds centre stage in deciding innocence or guilt. So it is no surprise that rules have been drawn up over the centuries as to how it should operate and as to who can or cannot sit. As Lord Bingham of Cornhill, the senior law lord, put it last week: “To a large extent the integrity of the trial process hangs on the jury’s integrity.” The rules, he added, reflect a truth: “If a metal bell be flawed, it will not ring true.”

The middle-class opt-out from jury service was one big flaw of the system. Juries, it was said, were dominated by the unemployed and by housewives. They no longer represented the population at large.

So in 2003 under the Criminal Justice Act the Government brought in a law aimed at ending the exemptions enjoyed by a range of people — judges, lawyers, doctors, priests — while tightening the rules to make it harder to get out of jury service. Since, scores of judges — including members of the Court of Appeal — and lawyers, including Cherie Booth, QC, have sat as jurors.

But do they influence the juries on which they sit — and does having a police officer or lawyer make that jury less impartial? Last week in the House of Lords three defendants challenged their convictions on the basis that their right to a fair trial was breached because the juries included, in two cases, police officers and in the third, a lawyer employed by the Crown Prosecution Service (CPS).

The five law lords were divided but a majority upheld appeals in two of the cases — one involving a man charged with two rapes whose conviction has been quashed and his case sent back to the Court of Appeal — because of the possibility that the defendants had an unfair trial.

The ruling raises questions about the workings of the rule obliging lawyers and police officers to serve as jurors. The reason that lawyers and others were originally exempt was examined by a committee under Lord Morris of Borth-y-Gest that reported in 1965. It was right, the committee said, that those whose work was “connected with the detection of crime and the enforcement of law and order must be excluded, as must those who professionally practise the law, or whose work is concerned with the functioning of the courts”.

One danger, the committee added, was that their specialist knowledge may cause them to be “built-in” leaders on a jury. Even civilian employees of police forces “became identified with the service through their everyday contact with its members”. Police officers are still barred from sitting in Scotland, Northern Ireland, Australia, New Zealand, Canada, Hong Kong, Gibraltar and various US states.

The issue was next looked at by Lord Justice Auld in his review of the criminal courts published in 2001. He took a different view. He said that there was no reason why the risk of prejudice was any greater among those excluded than with anyone else sitting as a juror — shopkeepers or house-owners who had been burgled, or car owners whose cars had been vandalised or people with strong views on legalising drugs or euthanasia.

Risk of prejudice or partiality could never be eliminated, he said. “The variety of prejudices that jurors can have are almost unlimited.” But provided the judge was satisfied that a potential juror was not likely to raise a reasonable suspicion or apprehension of bias, then the fairness of the trial should not be at risk.

In his ruling last week, Lord Bingham cited Auld — whose views were echoed in 2005 by the Court of Appeal — that the system operates its own checks and balances. The fact that there are 12 jurors was a protection against the prejudices of any individual resulting in unfairness, he said, dismissing the first appeal in which it had emerged that the jury foreman was a police officer.

But the second case was different. In it, a police officer on the jury worked in the same borough as the sergeant in the case, although they did not know each other. That in itself would not have been enough to have justified quashing the conviction. But the case turned on disputed evidence from a police sergeant.

So, Lord Bingham said, it did not involve the “ordinary prejudices and predilections to which we are all prone but the possibility of bias (possibly unconscious) . . . which inevitably flows from the presence in a jury of persons professionally committed to one side only of an adversarial trial process”, not just involved in some capacity in the administration of justice. An observer would say that the instinct of a police officer would be to prefer the evidence of a fellow officer rather than a drug addict — a “real and possible source of unfairness”.

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