The report, published by the miscarriage of justice charity Appeal, is the result of six months of observations at the Old Bailey, where Dr Nisha Waller and Tehreem Sultan followed 17 trials involving charges of murder or attempted murder under joint enterprise – a legal doctrine allowing individuals to be convicted for crimes they did not personally commit, if they are deemed to have assisted or encouraged the principal offender.

Their findings suggest that prosecutors are drawing what they term an “unreasonably wide net”, with defendants charged on the basis of tenuous or speculative evidence. In some cases, they said, the evidence was “weak, and in some, virtually non-existent”. One trial was cited as “especially concerning”, in which seven boys were charged with murder. In five of those cases, the prosecution relied almost entirely on the defendants’ “voluntary presence” at the scene.

The researchers criticised what they described as overzealous decisions by the Crown Prosecution Service and a failure to weed out weak cases at an early stage. “Defendants were charged with serious offences on weak evidential grounds which stimulated a range of problematic prosecution strategies, aimed at maximising the chances of conviction,” they said.

The report also highlights ongoing concerns about racial disparities in joint enterprise prosecutions. Of the defendants observed, 60 per cent were black and 79 per cent came from minority ethnic backgrounds. The authors said prosecution narratives often invoked racial stereotypes about gangs, despite commitments from the CPS to reduce such bias.

The only trial in which the majority of defendants were white was also the only one where nearly all were granted bail. Despite the severity of the charges, only eight of the 33 defendants identified as secondary parties were ultimately convicted of murder or attempted murder. In one case involving 50 charges against eight defendants, the jury returned a single conviction.

The findings echo earlier research by the Centre for Crime and Justice Studies, which showed that secondary suspects charged with murder had a conviction rate of 40 per cent, compared with a rate of roughly 60 per cent across all murder indictments.

The authors argue that many of the cases were built on conjecture rather than evidence, with prosecutors routinely assuming knowledge or intention – central elements of secondary liability – without proving them.

In a foreword to the report, Keir Monteith KC described the findings as “seismic” and said they demonstrated institutional racism in the criminal justice system. He added: “The CPS must stop prosecuting young black men just because they are voluntarily present somewhere near the scene. Wrongly prosecuting these young men for crimes they did not commit is immoral, contrary to the rule of law and costs millions. It has got to stop.”

A spokesperson for the Crown Prosecution Service said: “The CPS carefully monitors joint enterprise prosecutions, with senior legal oversight over every case to ensure that our approach is fair and proportionate.”