Britney Spears has dramatically impressed her trainer by losing 15 pounds (6.8 kilograms) in four weeks.

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

The Toxic 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 Life & Style: "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."

Maybe she can give pregnant sister Jamie Lynn some weight loss advice.

Check out Brit Brit's tight hot body here.

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