File photo: Cyber-security expert Dr Jessica Barker told Cheltenham Science Festival girls are also often contacted by bogus "model agencies" conducting such scams.

Children are being targeted by criminal gangs in online "sextortion" scams, experts have warned.

They say blackmailers befriend teenagers online, convince them to send explicit pictures of themselves and then demand money while threatening to make the photos public.

Cyber-security expert Dr Jessica Barker told Cheltenham Science Festival girls are also often contacted by bogus "model agencies" conducting such scams.

NSPCC figures presented at the festival show a third of schoolchildren have added a stranger as a "friend" online.

Last year almost 900 people were victims of webcam scams where intimate pictures are recorded online and used for extortion, according to Britain's National Crime Agency. 

At least four male suicides have been linked to the scams.