A judge has found Craig Pickering, a former special constable at the London courthouse, guilty of assaulting a prisoner.

Pickering, a 16-year veteran with London police, was accused of assaulting Timothy Van Dusen in the holding cells under the courthouse in May 2013.

Van Dusen was injured in the altercation, but the defence had argued that the court officer used reasonable force.

Van Dusen has acknowledged he has a history of mischief and violence, including assaulting and threatening a peace officer.

But in delivering his verdict Justice Lloyd Dean said he had issues with the testimony of most of the court officers, including the accused.

"He was in handcuffs and leg irons. What could Van Dusen do? How was he a danger to anyone at that moment?"

While Pickering was by all accounts good at his job, Dean said the surveillance video showed he went too far when handling a difficult prisoner.

Pickering had argued that Van Dusen was resisting, but that he was only trying to put him in the cell far from the door and that the injuries were accidental.

But the judge said "I don't accept his evidence on that most important point...The intention...was to drive  Mr. Van Dusen into the wall."

Pickering was let go by London police shortly after the incident and a civil suit filed by Van Dusen against the London Police Service was settled in April.