This month she gave a glimpse of her enviably toned legs while watching her husband, Prince William, on the polo field. And now the Duchess of Cambridge’s purse-friendly fitness secret has been revealed.

According to reports a £13 (about R170) workout consisting of a gym ball and a pair of dumbbells is behind the 30-year-old’s slender physique. During a daily 60 minute regime Kate – who also enjoys hockey and running – is said to use the simple equipment to keep her limbs toned to perfection.

‘Kate does an hour's workout every morning using the gym ball and dumbbells, which includes some core exercises like squats, sit-ups and arm curls,’ a source told Now Magazine. 'Kate knows how much scrutiny she'll be under at these very public occasions, so she's keen to look as slim and fit as she possibly can.’

Meanwhile Kate's sister, Pippa Middleton is a fan of Pilates and regularly visits West London-based instructor Margot Campbell.

Miss Campbell's interpretation of Pilates’s system (which she calls Pilates on the Go) adds a cardio element to the moves, boosting the effects in such a way that she has accrued a loyal fan base.

In a testimonial on the fitness centre's website, Pippa, who started visiting in 2009, states: 'As someone who is always “on the go”, Pilates has been a wonderful escape from my busy lifestyle in London.

'From breathing techniques, muscle toning to overall flexibility and relaxation, my Pilates sessions have become something of a weekly necessity that keeps me fit, happy and energised.'

Born and raised in East Kilbride, Scotland Miss Campbell adds: ‘You need to work the glutes – those are the gluteal muscles in your bottom – with squats, and the side leg series of movements.

'Lunges are really good too for lengthening and slimming the leg. If you follow the bottom and thigh work-out in my book – and keep it up – you will definitely notice a difference.'

Originally written by Sadie Whitelocks, this article appeared in The Daily Mail.