The Sussex Police pair in England who are both married to other people, are due before a professional standards panel next week.
Chief Insp Leet, an East Sussex commander, is accused of an inappropriate relationship on duty.
He also faces claims he had a sexual relationship with a member of the public between 2014-16.
Sgt Porter, a neighbourhood cop then on a roads policing team, is accused of missing a call to a fatal smash because she was with Leet.
She is also said to have asked a witness to make a statement to help her.
Sussex Police released claims against the Chief Inspector, who is based at a police headquarters in Lewes, East Sussex.