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PC Lee Birch, 30, zapped Daniel Dove with the 50,000-volt weapon after the well-worn boxers were flicked into his face.
The Chief Constable of Wilshire Police had tried to block the footage being made public after PC Birch was acquitted of assault.
But last night it was leaked to ITV News.
Mr Dove, 23, had been arrested for being drunk and disorderly outside a nightclub and was ordered to strip naked for a search once inside a police cell.
But he felt "humiliated" at the way he was being treated and hurled his boxers in a "split second" fit of temper.