Sources at HMP Wandsworth believe the mystery outbreak which has infected 370 inmates “could be swine flu”.

Many inmates complained of flu-like symptoms days earlier before they began suffering from chronic diarrhoea, it has been claimed.

It is not yet clear what caused the illness. The Prison Service confirmed Environmental Health officers had been sent to the prison to determine the cause of the outbreak.

Staff who had not been at the prison for 24 hours were sent home on Thursday morning.

A source said: “The first symptoms they had were flu-like ones. Inmates were complaining of feeling really achy and they had headaches. Then the diarrhoea started.

“I saw one inmate on Tuesday who looked dreadful. He was sweaty, white as a sheet and kept complaining that people weren’t taking any notice.

“The next day he was so ill he couldn’t leave his cell. That’s when I heard loads of other inmates had come down with it.

“I know someone who had swine flu about a month ago and their symptoms were the same. I wonder if it could be swine flu.”

The Prison Service refused to confirm what measures being taken to stop the spread of the illness, such as the restriction of visitation.

Earlier today a spokesman confirmed public health officials were going in to the prison to determine the cause of the outbreak.