PETERBOROUGH, Ont. - Ontario's police watchdog has charged a Peterborough constable with sexual assault in an incident involving a 40-year-old woman.

The Special Investigations Unit says the alleged incident took place on January 24, while the officer was off duty.

The SIU says Const. Christopher Robertson was charged following a four-month investigation triggered by a complaint.

The agency says it cannot comment further because the matter is now before the courts.

The SIU is an arm's-length agency that investigates reports of death, serious injury or sexual assault involving police.