Naomi Watts’ personal trainer has revealed how the actress undertook on a gruelling workout regime in order to transform herself into Princess Diana for the upcoming movie Diana.

New York-based fitness guru Simone De La Rue told Health & Fitness magazine how she got Naomi Watts into shape using her tried and tested methods of resistance training combined with danced-based cardio.

Simone De La Rue, who also counts Sandra Bullock, Matt Damon and Anne Hathaway as her clients, is famed for her tough yet effective dance work-outs.

She explained how she works with clients like Naomi Watts for three months to achieve the best results.

Simone De La Rue got Naomi Watts into shape using her tried and tested methods of resistance training combined with danced-based cardio

Simone De La Rue, an ex-professional dancer said: “When working towards a specific goal with a client, I encourage them to train five or more times a week, for one hour a day.”

“I set a programme that constantly challenges my clients and, during the last three weeks, we really increase the intensity.”

Simone De La Rue, 38, added that for her role as Princess Diana, Naomi Watts, 44, carried out a great deal of cardio work but the dance element makes it “a fun way to get your sweat on and burn calories”.

The fitness guru, who is originally from Australia, says her core-strengthening sessions, which incorporate pilates, yoga and dance, have become a favorite with celebrities because they deliver “long and lean” results.

One of her most famous clients is Sandra Bullock, who turned to the Body By Simone (BBS) workouts after she split from husband Jesse James in 2011.

Simone De La Rue revealed: “Sandy wanted to focus on her upper arms so we used wrist weights and slowly moved the arms in a move that mimics the <<Port De Bras>>.”

A personal session at her studio BBS studio in New York would set you back around $175, but group sessions cost a more reasonable $35.