In the space of 10 days, the hospital, in London Road, has been broken into three times and canisters of Entonox gas, which is used to combat pain relief, has been stolen.

At some point overnight between Saturday, June 26 and Sunday, June 27, the hospital was targeted for the third time, as thieves scaled the fence, closest to the A30 and gained access to the site. Once in they used bolt cutters to cut the padlock to one of the hospital's storage buildings, and took several canisters of the gas.

Police are concerned that the anaesthetic gas, which contains nitrous oxide, other wise known as 'laughing gas' is now being used to get people high.

Spelthorne neighbourhood inspector Sarah Greenhalgh, said: "The gas in the cylinders is used for pain relief. There is a suggestion that this is being used at parties and nightclubs because it gives people a high. We're worried of it getting into the wrong hands, especially if it is being used for leisure and recreational purposes. If someone uses it and mixes it with alcohol it can be very dangerous. We want to know if anyone knows someone that is able to get hold of this gas."

Canisters were also taken when the hospital was targeted two nights in a row between Wednesday, June 16 and Friday, June 18.

Entonox gas is a mix of nitrous oxide and oxygen and is frequently used by midwives during childbirth and by paramedics in emergency situations. Users get a high by inhaling the nitrous oxide which gives them a euphoric feeling.

Clinical director for theatres and anaesthetics, consultant anaesthetics Dr Michael Imrie, said: "Nitrous Oxide can be very dangerous and although it is referred to as the 'laughing gas' it is anything buy funny. It is normally used as a mild analgesic pain killer or an anaesthetic gas in operating theatres, under the strictest controls. In the wrong hands, misused, it can cause death."

Police are appealing for witnesses to the thefts.