If you're not already pounding the streets with a pair of FitFlops, you'll have to join the thousands in the queue for this summer's must-have footwear.

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 say that the specially-engineered sandals absorb shock and reduce the danger of sufferings 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.

The idea for the FitFlops came from Marcia Kilgore, founder of health spa company Bliss Spas. She wanted a summer alternative to MBT(Masai Barefoot Technology), shoes.

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

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

"Conventional footwear is designed to keep the foot stable so the muscles are more relaxed.

"The FitFlop destabilizes the foot slightly, creating a more continuous tension in the supporting muscles of the foot and leg.

"Every step you take in your FitFlops improves your core muscle strength and encourages better posture and burns calories.