The Ministry of Justice published data which showed black and minority ethnic (BME) jurors were "significantly less likely" to convict a black or Asian defendant than a white defendant in certain types of case.

Researchers said this so-called "same race leniency" appeared to reflect a belief among black and Asian jurors that the courts treat defendants more harshly if they are from an ethnic minority.

Jurors attempted to compensate for this perceived bias in the criminal justice system in a bid to provide a "level playing field", the report suggested. Such bias took place largely in cases where race was not a factor in the prosecution, it added.

"White jurors showed some same race leniency towards white defendants, but again this was only present in cases where race was not an explicit element of the case," the report said. "Both black and Asian jurors showed leniency for the black defendant, but there was no evidence of leniency for the Asian defendant by either Asian or black jurors."

The study involved 27 simulated trials where the racial make-up of the defendant, victim and jurors was varied and the jury results analysed.

The report said this tendency towards individual bias did not influence the outcome of the jury overall. "The case was tried 27 times and there were no significant differences in jury verdicts based on the race of the defendant," it said.

However, it noted that conviction rates at one London court where there was a high proportion of black and Asian jurors - usually three or four out of 12 - were low compared with juries in other parts of the country where there was perhaps one ethnic minority juror on each panel.

It suggested that "different social dynamics" on a jury may affect its decision-making.

Trevor Phillips of the Commission for Racial Equality said at the launch of the paper in central London: "The system can have biases even though it's populated by people of great good will. In this case, one of the biases clearly emerges because most jurors think the system is geared against black defendants. This is a bias we want to avoid as much as we want to avoid a bias in favour of white defendants. All biases should be ironed out."