The innovative flip flops, dubbed 'FitFlops', are designed to help tone and trim your legs by engaging muscles for a longer period of time with each step.

Designers claim that the "multi-density midsole" focuses on thighs, calves and glutes.

And they say that the specially-engineered sandals also absorb shock and reduce the danger of suffering joint strain.

The King's Road Sporting Club (KRSC) in London, which was one of the first stores to stock FitFlops, sold more than 700 pairs in the first three days.

"In 13 years of running this shop I've never seen anything like this," said owner Michael Conitzer. "It is the fastest-selling product we've ever had.

"We've had a waiting list ever since we got them in - we've even had to take them off the website. It's the product of the summer without a doubt."

The idea for the FitFlops came from Marcia Kilgore, founder of health spa company Bliss Spas. She wanted a summer alternative to MTB shoes - the shoes that mimic the effect of walking barefoot.

The science came from biomechanical engineers Dr David Cook and Darren James from the Centre for Human Performance at the South Bank University in London.

A spokeswoman for FitFlop said: "We knew the FitFlops thing would be big but we're having to fight to make more and keep everyone happy.

"Every step you take in the FitFlop helps tone and trim your legs.

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