A rise in reports of teenagers inhaling laughing gas in parks and public places has prompted a police warning.

Officers have warned children and their parents against sniffing any small metal cannisters they find in public spaces as these are likely to have been left over by people using laughing gas in the area.

Nitrous oxide, commonly known as laughing gas and dubbed 'hippy crack' by the Daily Mail, is not a controlled drug and its use is not illegal.

But medical experts have warned against using it recreationally as inhaling large amounts quickly can be fatal as it forces oxygen out of the lungs.

In recent weeks, police have been alerted to several incidents in Stanley Road, Carshalton, and The Mount, Coulsdon.

Police believe the driver of a white car spotted in the Mount has been supplying laughing gas to youths and are keen to speak to the person.

Police were called by a woman who found several small, silver cannisters next to where her car was parked in Sutton yesterday. Officers also responded after a person spotted a driver throw an empty box of nitrous oxide cannisters from the window of a car.

The warning on the box stated: "Do not use for any other purpose. Misuse can be dangerous to your health. Do not inhale."

Inspector Ian Hicks said: "Although it’s not illegal, that doesn’t mean it’s safe to inhale and we would urge children not to put their lives at risk by doing so.

"We would also encourage parents to make themselves aware of the dangers of nitrous oxide and to talk to their children about the risks."

It is illegal in England and Wales for anyone to sell nitrous oxide to children under 18 if they believe the children will be inhaling it.