Joint enterprise cases in England and Wales have soared over the past four decades, according to a report calling for a change in the law so that individuals are held accountable only for their own actions.

The Centre for Crime and Justice Studies (CCJS) also found sentences have become harsher under the legal doctrine, which allows for individuals to be convicted of crimes they did not physically carry out if they were present at the scene or held an association with the principal culprit.

The report claims prosecutors have drifted from a common-sense distinction between those who have caused a death and those who may be culpable only for lesser offences, resulting in a “job lot” prosecution approach, which means bit-part players are convicted of the serious crimes of others.

The authors found that the number of homicide cases involving three or more defendants had tripled between 1984 and 2024, from 18 to 54 cases a year, and now account for nearly 10% of all homicide prosecutions.

Excluding life sentences, 42% of defendants convicted of manslaughter in multi-defendant cases in which they were not considered the main suspect were found to have received a prison sentence of more than 10 years in 2022, compared with 7% in 2012. One of the CCJS’s recommendations is that there should be a separate, less harsh, sentencing framework for secondary parties.

Helen Mills, the director of programmes at the CCJS and the report’s co-author, said: “There are clear limits to what can be achieved through legal challenges and individual appeals alone. With the Law Commission currently reviewing homicide laws and the government expressing a commitment to fixing a broken justice system, there is a unique opportunity to move beyond the current logjam. In place of an unjust, job lot approach, we need some nuance, making sure people are held to account for what they actually did.”

The report says that the prosecutorial approach, as well as the law, needs changing given that multi-suspect homicide cases were described as “exceptional” in the 1960s, but are now a systematic norm with defendants being disproportionately young, male and black.

The report found that about 40% of those convicted in homicide cases involving four or more people are aged 18 to 24. While convictions of children for murder are rare, since 2010 just over half of all children under 16 convicted of murder were in multi-defendant cases in which they were not considered the main suspect – the highest proportion of any age group – according to the report.

The CCJS found that black people were three times more likely than white people to be convicted in group cases of four or more defendants, echoing the Crown Prosecution Service’s own joint enterprise monitoring data, published last year.

The authors identify three priorities for potential reform. They are: narrowing the scope of the law through a workable test that specifies when someone can be held liable for the crimes of another; a separate sentencing framework for secondary parties to ensure proportionate sentencing; and greater transparency, requiring prosecutors to state each defendant’s individual conduct and intent and identify possible alternative charging.

A 2016 supreme court judgment found that the law had been wrongly applied for more than 30 years and the bar set too low with respect to the required intent of any secondary co-accused, which led to hope of a course correction, but the CCJS says it has had no sustained impact on the number of multi-defendant homicide prosecutions.

A Ministry of Justice spokesperson said: “We are aware of the concerns on joint enterprise which is why we are keeping the law under review.

“There are currently several ongoing reviews in this area and any reforms should be informed by their findings.”