A prisoner who killed a fellow inmate with a single punch has been jailed for seven years for manslaughter.
James Johnson, 38, punched 39-year-old Luke Derriman with such force inside HMP Hewell, Worcestershire, that he sliced an artery in his neck.
Johnson, of Tazmania Close, Cheltenham, said he acted in self-defence after Mr Derriman was aggressive.
Jurors at Worcester Crown Court heard it took prison staff 15 minutes to realise there had been an altercation.
Johnson attacked Mr Derriman, a father-of-two from Slough, on 23 January when the prison had opened all the cell doors to allow inmates to "socialise".
He went downstairs to Mr Derriman's cell and fatally punched him after the victim put a hand on his shoulder.
Passing sentence, Judge Robert Juckes said Johnson's actions had been "catastrophic".
"It had a catastrophic impact. You are a well powerful, well-built man, you must have known that," he said.
The court heard there was "chaos" in the prison at the time, with another fight happening on the floor directly above Johnson and Mr Derriman, as well as paper being thrown from above.
Prison staff described finding Mr Derriman unresponsive in his cell, but there had not been an officer nearby at the time of the attack. He died later the same day in hospital.
His mother Sandra Conway said Mr Derriman "touched the lives of everyone who knew him" and was a "caring, loving son and father".
A report in 2017 into HMP Hewell found there had been 49 fights in a six-month period and that violence levels were "far too high".
It took the jury less than one hour to find Johnson guilty of manslaughter after seeing CCTV footage of the altercation.
An independent investigation into Mr Derriman's death by the Prisons and Probation Ombudsman is ongoing.
A Prison Service spokesperson said it would be "inappropriate to comment" until the investigation is complete.