Imagine you are an Asian man on trial for arson, facing the prospect of years in jail if you're found guilty. Your fate is in the hands of 12 men and women chosen at random from the electoral roll. But what if some of those jurors are racist? Suppose they make disparaging remarks in the jury room about your appearance, your accent, your poor English and your business integrity. Comments such as "They're all the same. I've worked with them in Birmingham. I know what they're like." Or "The only reason he's got a clean record is because he's never been caught," and "He would have been better off staying in India."

That's what happened during Sajid Qureshi's trial at Mold crown court in October 2000, according to one of the jurors who convicted him by an 11-1 majority and saw him sent down for four years. And that's not all. In a letter to the court six days after his conviction, she claimed that some of her fellow jurors seemed to have already decided their verdict from the start, that one juror fell asleep during the evidence, that another was deaf and that some tried to bully others. Enough, you might think, to raise serious doubts about whether Qureshi had a fair trial.

But not enough for the court of appeal, where he never even got to first base. Last February, the court refused him permission even to launch an appeal. The stumbling block was the Contempt of Court Act 1981, which bans anyone - and that includes judges - from inquiring into the secrets of the jury room. In Qureshi's case, the judges said, the inquiries already made had gone too far. If trial by jury was to be preserved, allegations could be investigated only while the trial was still going on. Once it had ended, however outrageously biased the jurors were, however improperly they behaved, the defendant had no redress.

British society is riddled with racism, Sir David Calvert-Smith, director of public prosecutions, contends in an interview to be broadcast today on Radio 4's On the Ropes. The police, the crown prosecution service and the prison service have all admitted institutional racism. A study of West Midlands courts by Oxford University in 1989 found that seven judges were sentencing blacks much more harshly than whites. If so much of the criminal justice system is racist, why should the jury be any different?

Yet, while inquiry after inquiry investigates possible racism elsewhere in the system and makes strenuous efforts to weed it out, no one can inquire into the prejudices of juries.

John Spencer, professor of law at Cambridge University, argues in the forthcoming July issue of the Cambridge Law Journal: "If juries are composed of 12 people chosen from the electoral roll at random, it is inevitable that they will sometimes be dominated by people who are racists, or are irresponsible and silly. Our legal system is gravely deficient if it fails to guard against this obvious danger."

Last month, the court of appeal refused to look into allegations by jurors in two other cases where fellow jurors were accused of bias. A juror in the trial of an Asian man for a minor indecent assault on his stepdaughter wrote to the judge after he was convicted. The allegation was that fellow jurors had insisted that the man, who gave evidence through an interpreter, understood English perfectly well but had asked for an interpreter to gain more time to answer prosecution questions. The fact that he had asked for an interpreter when he didn't need one showed he was a liar, they said. The second appeal, dismissed on the same day, was brought by two men convicted of wounding with intent. They appealed after finding out that a juror had written to the judge claiming other jurors had reasoned that since they could not decide which of the men had wielded the weapon they had better convict both. According to the whistle-blower, fellow jurors had suggested that life was unfair and innocent people sometimes had to be convicted. The appeal court held that it was barred from investigating the jurors' stories by a 1997 judgment in the case of Badrul Miah, an Asian youth convicted of murdering a white teenager in north London. In last month's cases, the judges refused permission to appeal to the House of Lords, but stated that the cases raised a question of law of public importance - did the ban on questioning jurors clash with the right to a fair trial in article 6 of the European convention on human rights? - leaving it to the lords to decide whether to hear a further appeal.

The ban's potential for wrongful convictions did not escape the appeal court judge Sir Robin Auld when he drew up his blueprint last year for radical reform of the criminal justice system. His recommendations will form the major part of a criminal justice white paper next month and a bill in the autumn. The ban, Sir Robin argued, was "indefensible and capable of causing serious injustice" and highly vulnerable to a challenge under article 6. He recommended that the curtain should be lifted far enough to allow trial judges or the appeal court to look into allegations of bias or improper behaviour by jurors which could have led to an unfair trial.

The government has adopted many of Sir Robin's recommendations, particularly those designed to "re-balance" the system in favour of victims and against defendants. Some of the measures - an end to the double jeopardy rule, more information about defendants' previous convictions, more hearsay evidence - have been heavily trailed. But there are no signs that ministers plan to allow judges a foot in the jury-room door. The lord chief justice, Lord Woolf, a powerful voice in the criminal justice debate, told the Guardian last week that he would be reluctant to allow judges to investigate jury deliberations.

Professor Spencer, one of 12 consultants appointed by the lord chancellor to the Auld review, takes issue with that stance. He describes the ban as "quite unacceptable", adding: "I think, as Robin Auld said, this involves a real risk of genuine miscarriages of justice. And even if it would involve cost and trouble, and even though there would be false allegations for every genuine allegation, nevertheless if people are risking long terms of imprisonment for things they didn't do in consequence of it, it shouldn't be permitted."

In his Cambridge Law Journal article, Spencer argues that the court of appeal's decision in Qureshi's case is "to put it mildly, questionable". He says the ruling is "almost certainly incompatible with article 6 of the European convention on human rights, which guarantees defendants a 'fair hearing' before a tribunal that is 'independent and impartial' ". More fundamentally, he adds, "the refusal of the legal system to investigate a serious allegation of jury bias or misconduct is objectionable because it amounts to a refusal to intervene where there is a serious risk that the defendant was convicted, not because the jury thought him guilty, but because it did not like him". Spencer argues that the danger of false complaints can't justify ignoring them all, since we happen to know that some allegations are true. In one recent case it emerged that some jurors had used a ouija board to try to contact the murder victim to find out who killed him.

The court was able to look into the allegation, verify its truth and quash the conviction only because the jurors had done it in the hotel where they were staying overnight and not in the jury room. In 1997 the European court of human rights in Strasbourg rejected a complaint by Badrul Miah that he was denied a fair trial because of jury bias. A document stating that the jurors had made up their minds about his guilt before hearing the evidence was handed to Miah's solicitor more than a year after the trial by the partner of one of the jurors. The court felt the allegations were vague and noted that no jurors were accused of making racist comments. Since then, though, the Strasbourg court has ruled that an Algerian standing trial in France was denied a fair trial when the judge allowed a juror who announced in advance that he was racist to serve on the jury; as was a British-born Asian charged with fraud when the judge refused to discharge the jury after a juror admitted telling a racist joke.

Spencer calls for urgent action to prevent future miscarriages of justice and puts forward three options: lift the ban and let the court investigate; have the judge sit with the jury during their deliberations; or tape-record jury deliberations. Auld rejects letting the judge sit with the jury, which happens in France, as "alien to our criminal process". Spencer's preferred option is tape-recording; he suggests that the tape could be sealed and locked away, to be opened if plausible allegations of jury misconduct surface later. "Taping the discussions would make the jury take care in all cases and it would mean there was some valid record.

"The police fought the taping of interviews but they gave in, and the result is that we have confidence in police interviewing suspects in a way we never did before. I think we would end up with greater confidence in jury trial because we would know, if it derailed badly, there would be some means of checking."