Humphrey Burke, 28, from Oxford, kicked the 54-year-old grandmother, from Harold Wood, in the head at Blackfriars Crown Court whilst she was doing her duty.
He had denied murder but pleaded guilty to manslaughter on the grounds of diminished responsibility during a hearing at the Old Bailey today (Thursday, December 23).
Judge Mark Lucraft QC adjourned sentencing until January 11 and Burke has been remanded in custody in a secure hospital.
The Crown Prosecution Service accepted Burke’s guilty plea to manslaughter, with prosecutor Tom Little QC telling the court: “Our position is a plea that relies on the partial defence of diminished responsibility would be acceptable to the Crown.”
In 2016, Burke - a former law student - was ruled unfit for trial due to mental illness and he was handed a hospital order following a trial of the facts.
At the time of the attack, a psychiatric report had been prepared but no definitive diagnosis had been made.
A grandmother twice over, she had worked as a custody officer for Serco for over 10 years and was reportedly the first prison custody officer to die in the line of duty.
Gia Sofokli, who had been shadowing Barwell on her first day in the job, described the attack to the Old Bailey during the previous trial.
She had said: “He just literally pulled his leg back and kicked her.
“She screamed quite loud and then it was about two seconds before he swung again with his foot but this time it got her in the face.
“Her head went back but it did not smack the floor. It just snapped back.”
Security contractor Serco is due to stand trial for breaching health and safety laws in relation to the death in May next year. It has denied the charge.