"Imagine you are on a mission to get home and feed the baby!" yells Stuart as she strides off ahead off the class.

The mothers then complete a series of circuit-training based exercises. They use their babies as weights for shoulder lifts and bicep curls, and lie them on their backs on the grass and kiss them each time they dip on a press-up.

At any point, mums can drop out for a few minutes to feed or comfort a wailing baby.

"Older babies seem to find powerpramming fascinating and hilarious," said Stuart. "The younger ones tend to find the movement makes them sleepy and they often stay in their prams throughout the workout."

Rebecca Maxwell-Hyslop, whose twins Ben and Alice are just 11-weeks-old, is a new convert to "powerpramming".

"It's such a fabulous idea," she says, breathless but happy after her hour-long workout.

"If I had to go to the gym and put the twins into a creche, it would cost me a fortune. This way we all get some fresh air, I get some exercise and my babies have slept all the way through."

Since starting "powerpramming" in London in 2004, Stuart has been contacted by eager mothers from across Britain and in Spain, Japan, New Zealand and the United States.

"I have now set up a workout called 'opensource powerpramming' that I send via e-mail to people in this country and abroad so they can set up groups on their own," she said.

American Margaret Meldrum, who joined the "powerpramming" group with a seven-week old daughter Olivia, said it was a concept that would do well in the United States.

Meldrum says she had found it hard to find a post-natal exercise class in London that was rigorous enough for her tastes until now.

"It was a proper workout," she said. "It wasn't just a walk in the park."