A police officer is facing the sack after he was caught on CCTV throwing a woman into a cell, badly injuring her.
The footage also shows Sgt Mark Andrews dragging Pamela Somerville through Melksham police station in Wiltshire.
Ms Somerville, 59, was arrested in July 2008 after being found asleep in her car. Her treatment left her with facial injuries and needing hospital care.
Andrews has been convicted of causing the woman actual bodily harm and is expected to be dismissed from his job.
Ms Somerville had been detained for failing to provide a sample for a breath test.
She denies refusing to provide a sample and said the charges against her were later dropped.
She said: "I thought I was going to die, by people that you trust.
"You can't imagine it. They're the people that you trust, so who do you trust after that?
"You don't treat somebody - even if you've found them guilty of the worst possible thing you can possibly imagine - you don't put them in a situation where they have to be in hospital twice in one day, do you?"
The case was brought after one of Andrews' colleagues reported the matter to senior officers, who have apologised to Ms Somerville.
Wiltshire Police's Assistant Chief Constable Patrick Geenty said it was a "completely disgraceful incident".
"We are very concerned when anyone is injured in our custody and the court has decided that this injury was as a result of a criminal assault by Sgt Mark Andrews, a member of Wiltshire Police who was performing duty as a custody sergeant at the time," he said.
"We respect the decision of the court and the force has formally apologised to the injured lady for the assault she suffered while in our care.
"People have a right to expect that the police will always act by placing the safety and welfare of the public as their first priority.
"This is particularly so when in police custody when irrespective of the reason for their detention, people should feel and be safe."
Andrews, who has been suspended on full pay since his conviction, is due to be sentenced for the assault at Oxford Magistrates' Court on Tuesday. He is also to face a police misconduct hearing.
BBC home affairs correspondent Steve Brodie said he understood that the officer would be sacked.