More than 30 witnesses who served with or alongside special forces soldiers broke their silence to the BBC’s Panorama programme and spoke about illegal killings and executions of detainees, including children, during the invasions of the two countries.

David Cameron – who was prime minister from June 2010 to November 2013, the period now under scrutiny by a judge-led public inquiry into special forces – was repeatedly made aware of concerns about night raids and killings raised by then-Afghan President Hamid Karzai, according to the BBC.

A spokesman for Cameron said “any suggestion that [he] colluded in covering up allegations of serious criminal wrongdoing is total nonsense”.

The special air service and the navy’s special boat service, the UK’s top special forces units, were at the centre of the testimonies.

“They handcuffed a young boy and shot him,” recalled one veteran who served with the elite soldiers in Afghanistan. ”He was clearly a child, not even close to fighting age.”

Killing of detainees “became routine”, the veteran said, adding that the soldiers would remove plastic handcuffs from executed detainees and plant weapons by their bodies to make it look like they were fighters in photographs taken from the scene.

Another veteran with the navy’s special forces regiment said some service members displayed “barbaric” and “psychopathic” behaviour as they felt untouchable by the law.

One former soldier described the killings as something that could turn “addictive” as some soldiers became “intoxicated by that feeling” in Afghanistan.

“On some operations, the troops would go into guesthouse-type buildings and kill everyone there,” he stated.

“They’d go in and shoot everyone sleeping there, on entry. It’s not justified, killing people in their sleep.”

Even wounded people who did not pose a threat to anyone were executed in breach of international law, witnesses said, speaking on condition of anonymity.

A former special forces operator said an execution of an unarmed person in Iraq was never properly investigated, adding that senior commanders were aware of the problem long before deploying to Afghanistan.

The BBC also obtained new video evidence that showed squadrons kept kill counts to compete with others.

One veteran stated a former colleague was trying to get kills on every single operation, having become “notorious” for killing dozens of people.

Another veteran added “everyone knew” about the killings in the UK special forces command with testimonies indicating officers would falsify postoperational reports to avoid scrutiny.