Britney Spears has left her trainer stunned by her work-out schedule, which has helped her shed 15 pounds in four weeks.

Marc Corliss insists the pop superstar is among his best ever clients because she does everything he tells her to do and works out on her own.

The pop star has become a regular at Hollywood’s Bally’s gym, where she often works out with her father Jamie, and Corliss is impressed by her work ethic.

He tells America’s Life & Style magazine, "She’s hell-bent on getting a rocking midsection again.

"She’s been training hard for the past four weeks, doing about 500 or so abdominal exercises every day. And she’s seeing a major difference already."

Corliss reveals Spears is also doing an hour of interval training, like sprinting mixed with weight training, four days a week.

He adds, "She’s a real go-getter, super upbeat and fun to be around. She’s motivated and positive; she’s definitely on the right track.

"I’m training her the way I would train an athlete. With her dance background, she’s already in better shape than most people."

The personal trainer also reveals that Spears likes to workout to her favourite radio shows on pop and hip-hop station 107.2, which often plays her hits.