A police officer kicked a handcuffed 15-year-old boy in the face leaving a colleague in "disbelief", a disciplinary panel has heard.
Following the attack in Marsh Street, Bristol, Adam Kitchener told the teenager: "Oops, sorry mate, I didn't see you there," the hearing was told.
He was jailed for 14 weeks last month after admitting the assault.
Chief Constable Andy Marsh ruled he would have been sacked for gross misconduct had he not already quit.
Police prosecutor Shahzal Hamid said the boy was arrested and placed in handcuffs before Kitchener, who worked for Avon and Somerset Police, used "excessive and unjustified force".
Kitchener, 36, who did not attend the hearing, kicked the youngster as he was on the ground, the hearing was told. It also heard the boy was "compliant" throughout his arrest.
Mr Hamid said another officer at the scene told the Independent Office for Police Conduct, which investigated the case, he was in "disbelief" and had "never seen such use of force in 10 years of policing".
Kitchener, who was a police officer for 10 years and joined Avon and Somerset in 2016, having previously worked for the Thames Valley and Metropolitan forces, said in a police interview he was going through a marriage breakdown at the time, which may have "influenced" his behaviour, the panel was told.
However, he accepted this did not excuse it.
Announcing his ruling, Mr Marsh said Kitchener's conduct was "entirely incompatible with the values of the force".
"The former officer would have been dismissed had he been serving today," he said.
The decision means the former officer can never work for the police again.