People laugh when they step into the offices of Jo Brooks PR to find a trio of agents perched precariously on exercise balls.

But giggles soon turn to requests for more information when they explain how they are keeping fit at their desks.

Their workout-while-you-work involves sitting on large inflatable Fit Balls, which ensure correct posture.

Staff at the agency in Gloucester Passage, Brighton, believe the idea will soon catch on and office balls will become as common as chairs.

Since fitness coach Tim Marshall introduced them as part of his workout regime, the trio say they have all lost weight.

They are now fitter and ready to cope with the demands of looking after a variety of clients, including TV presenters, models and business celebrities.

Jo Brooks, 34, who runs the agency, said: "When people walk in and find us sitting on our balls, they at first think it is very funny.

"Then when I explain about correct back posture and how it is helping us all to get fit, they get jealous and say 'I hope my boss gets me one'."

Jo brought in a fitness instructor when she became concerned about the amount of time staff spent hunched in front of their computer screens.

Her colleagues Alev Zahir and Ella Grover, both 23, were also worried about their health.

Along came Tim to offer his office-based workouts and help the trio become slimmer and fitter.

Jo said: "We all joked and laughed when Tim introduced us to the balls but they really work.

"Thousands of people take time off from work with back problems and I don't want that to happen to my staff.

"A healthy body makes for a healthy mind and a healthy work rate. If you feel well, you work well."

Tim visits the office twice a week to put the staff through their paces.

He said: "The balls force you to sit upright and strengthen your abdominal muscles. Bad posture leads to a lot of problems and sitting on the Fit Balls, along with the fitness regime I have devised, will help prevent problems in later life."

Alev said: "I have always had a bad posture but these balls are helping me. The only thing which annoys me is Ella constantly bouncing around on them.

"It is sometimes difficult to concentrate when the person opposite you is continually bobbing up and down."