The conviction of Barry George for the murder of Jill Dando has persuaded many against the case for allowing juries to see a defendant's criminal record.

The 11 Old Bailey jurors (the twelfth retired from the trial early) found George guilty largely on the basis of circumstantial evidence without ever knowing that he had once attempted to rape a woman or that he had been found by police in the grounds of Kensington Palace stalking Diana, Princess of Wales.

Nevertheless, the Government has made it clear that it wants to change the law, so that past convictions can become part of the case against the defendant.

It's a move that has provoked much anger from civil rights groups. After plans to erode the ancient principle of double jeopardy and to curb the citizen's right to a jury trial, many lawyers regard this latest attempt to tip the scales of justice in favour of the prosecution as a reform too far.

It's a long-established principle in this country that a defendant's guilt should not be decided on the basis of how many crimes they have committed in the past, but solely on the evidence which links the defendant to the offence for which they have been charged.

John Wadham, director of Liberty, describes the Government's proposal as "an attack on the fundamental presumption of innocence." He says: "Juries have to ask themselves whether there is enough evidence in each case to convict beyond reasonable doubt. Introducing details of previous convictions may lead juries to convict where there is insufficient evidence, and therefore lead to miscarriages of justice."

The Government knows that its latest big idea for the criminal justice system is one of its most controversial, and there is deep unease among senior ministers about how to proceed.

When this idea was first mooted last year, ministers were known to have expressed concern about how they were going to square telling a jury about a defendant's previous convictions with the requirements of a fair trial under the Human Rights Act. Accordingly, Labour's manifesto commitment to introduce the change is worded vaguely, and reflects little more than the previous Home Secretary's desire to see it come to fruition.

Jack Straw, keen to build on his reputation as a hard-line Home Secretary prepared to give criminals no quarter, is understood to have been influenced by a series of rape and paedophile cases, in which the jury only discovered after the acquittal that the defendant had a long list of previous convictions for similar sexual offences.

He must have taken soundings from the judiciary, some of whom are broadly supportive of the move. Gerald Butler, a retired crown court judge, says: "If the law is an ass then it is in the application of this rule. Where previous convictions are relevant and do not overly prejudice the defendant I think they should go in."

He says it's "ridiculous" that under the existing rule of evidence convictions can be adduced in certain circumstances, for example where the defendant accuses a witness of lying or asserts his own good character, but the jury cannot use this to make assumptions about his guilt.

"If a defendant has committed 20 burglaries before, then of course this is relevant to the fact that he might have committed another."

It was Mr Straw who first tested the public's appetite for such a reform by making room for the proposal in his 10-year crime plan, which was published earlier this year. Unsurprisingly, it met with general hostility but still found a place in Labour's manifesto, where the watered-down version is referred to as a "new presumption" in favour of allowing evidence of relevant previous convictions.

How far David Blunkett, the new Home Secretary, and the rest of the cabinet are prepared to go will depend largely on what Sir Robin Auld says in his review of the criminal courts, which is expected to be published in the autumn.

Sir Robin, a Court of Appeal judge, has been keen to explore the issue of previous convictions, and has taken soundings from lawyers, judges and others from the criminal justice system. He will also have studied the limited research on the effect that previous convictions tend to have on juries and magistrates when considering their verdicts.

The most comprehensive work to date was undertaken by the Law Commission, which published a paper on this subject in 1996. It provisionally recommended that the law should be clarified, so that any prejudicial evidence should only be allowed in cases where its probative value outweighs its likely prejudicial effect.

These conclusions were hardly a green light for radical reform. After all, this is the test that judges already use to exclude or admit prejudicial evidence during trials which take place under the current law. The rule is supposed to act as a safeguard against an unfair trial, but in effect it gives the judge a wide discretion over what is actually put before the jury.

As the Law Commission notes, in the trial of Rosemary West the judge used the existing law to admit previous evidence of her bad character, which must have had some influence on the jury. The Law Commission is to publish its final findings in October. The most recent research evaluating the impact on juries of evidence of bad character was published in 1995 by the Oxford Centre for Socio-Legal Studies. The report's author, Dr Sally Lloyd-Bostock, concluded that previous convictions affected the jury in a number of ways, most of them leading to prejudicial findings.

Certain types of conviction, such as indecent assault on a child, said Dr Lloyd-Bostock, can be particularly prejudicial, whatever the offence charged. This was also true of recent similar convictions, which she said increased the perceived probability of guilt. She also warned that a previous conviction for indecent assault on a child, because of "the all-round negative evaluation of such a person", will have a significant impact on the jurors' perception of the defendant's credibility as a witness.

But the research also showed that knowledge of a previous conviction for a dishonesty offence does not decrease the defendant's credibility as a witness in the eyes of jurors. Dr Lloyd-Bostock completed her second piece of research last year. This time, she looked at what happens when magistrates are told about previous convictions. Her findings mirrored her earlier study on juries, and they offered little comfort to those ministers who might have been hoping to use the research to justify proposals.

She concluded: "The results showed clearly that the magistrates' ratings of likely guilt were significantly affected by information about the defendant's prior record."

A more fruitful line of inquiry for Whitehall policy-makers might be comparisons with other jurisdictions. In America, there is much more flexibility to allow evidence of previous bad character to show that the defendant is guilty of the offence charged, rather than merely suggesting that he or she is an unreliable witness. In 1994, the US Congress passed the Violent Crime Control and Law Enforcement Act, which gives the prosecution more scope to introduce such evidence where a sexual crime has taken place. The Law Commission considered just such a proposal itself, but decided that a special case should not be made for sexual offences. If after Sir Robin Auld has reported, the Government is still determined to force some kind of change, it might consider other safeguards to prevent juries from simply convicting on the basis of previous behaviour.

Sir Robin is also known to be considering plans to make juries give reasons for their verdicts. If juries had to justify their decisions, as magistrates already do, there would be less danger of convicting for the wrong, or for prejudiced, reasons.

Without such a safeguard, there is a very real danger of a return to the bad old days of miscarriages of justice on a scale that hasn't been seen since the Seventies and Eighties, when the full scale of the quashings of the convictions of the Guildford Four and the Birmingham Six provoked public outrage.

See also:
The Auld Review - articles on the proposals put forward by Sir Robin Auld
Second time unlucky? - an article on the subject of double jeopardy