Corporal Donald Payne, 35, admitted the charge of inhumanely treating civilians, becoming the first member of Britain’s armed forces to admit to a war crime.
Payne, formerly of the Queen's Lancashire Regiment but now of the renamed Duke of Lancaster's Regiment, pleaded not guilty to charges of manslaughter and perverting the course of justice.
Six other defendants deny all charges against them.
The manslaughter charge relates to the death of Baha Musa, a 26-year-old hotel receptionist in the southern Iraqi city of Basra, who died in September 2003 whilst being held with six work colleagues by British soldiers.
Payne entered his plea as he and six other soldiers went on trial at a military court in Salisbury, southern England.
The Iraqi detainees were arrested in raids on several hotels in Basra by British soldiers following intelligence that the buildings were bases for Iraqi insurgents.
Julian Bevan, prosecuting, said the Iraqi detainees were "repeatedly beaten, kicked and punched when handcuffed and hooded ... made to maintain a stress position for an unacceptable length of time, deprived of sleep, continually shouted at and generally abused".
He told the court-martial that a post-mortem examination of Mousa's body revealed 93 different injuries, including fractured ribs and a broken nose. One civilian said he was photographed beside a soldier making a clenched fist, Bevan said.
Showing a map of the detention facilities to point out how close they were to other buildings of the military camp, Bevan suggested that other soldiers in the area likely heard the screams of the captives.
"The person in immediate charge was Corporal Payne"
Julian Bevan, prosecutor
"Shouting, bawling and screams from that facility over the ensuring 36 hours must have been heard by numerous soldiers, and indeed officers, in that camp and yet no one appears to have raised it as a concern," Bevan said.
"The person in immediate charge was Corporal Payne," he said.
Two of the other defendants are accused of war crimes in the first prosecution of British soldiers under the International Criminal Court Act (ICCA) 2001.