PA Sport Powell, at 31 the youngest head coach in Super League, makes way for the return of former Broncos boss Tony Rea on a temporary basis for the rest of the season.
Powell, who has been offered the chance to remain with the club, has paid the price for a dismal season for the Londoners, who are bottom of Super League with just three wins from their 20 matches.
Chairman David Hughes said: "We had to make changes because our performances were unacceptable. This is a result of our mid-season internal review with other findings due to be concluded soon."
