Britney Spears has embarked on a strict new diet and fitness regime.

The 33-year-old singer, who has sons Sean Preston, nine, and Jayden James, eight, with ex-husband Kevin Federline, was reportedly ''horrified'' when she saw untouched photographs of herself on the set of her 'Pretty Girls' music video in April and has since lost 12 pounds.

A source close to the star said: ''It was a big wake-up call and she's been religiously getting back into shape.''

The blonde beauty told pals she felt the ''biggest she's ever been'' at the time so she immediately amped up her workouts and banned junk food from her home.

But Britney thinks she will always have bad memories of shooting the video for the track with Australian rapper Iggy Azalea, which flopped earlier this year.

The insider told America's OK! magazine: ''She'll always have bad memories of that video because of the unflattering pap shots during filming. There was no way she was going to promote it after that - she was too mortified.''

The 'Toxic' hitmaker is clearly eager to show off her trimmer figure on the beach, as she flew to Hawaii with her sons on Wednesday (22.07.15).

The trio were spotted boarding a commercial flight in Los Angeles with a bodyguard.