Giving their accounts publicly for the first time, the veterans described seeing members of the SAS murder unarmed people in their sleep and execute handcuffed detainees, including children.
"They handcuffed a young boy and shot him," recalled one veteran who served with the SAS in Afghanistan. "He was clearly a child, not even close to fighting age."
Killing of detainees "became routine", the veteran said. "They'd search someone, handcuff them, then shoot them", before cutting off the plastic handcuffs used to restrain people and "planting a pistol" by the body, he said.
The new testimony includes allegations of war crimes stretching over more than a decade, far longer than the three years currently being examined by a judge-led public inquiry in the UK.
The SBS, the Royal Navy's elite special forces regiment, is also implicated for the first time in the most serious allegations - executions of unarmed and wounded people.
A veteran who served with the SBS said some troops had a "mob mentality", describing their behaviour on operations as "barbaric".
"I saw the quietest guys switch, show serious psychopathic traits," he said. "They were lawless. They felt untouchable."
Special Forces were deployed to Afghanistan to protect British troops from Taliban fighters and bombmakers. The conflict was a deadly one for members of the UK's armed forces – 457 lost their lives and thousands more were wounded.
BBC