A paedophile who used internet chat rooms to hack into the computers of schoolgirls and blackmail them into sending explicit pictures of themselves was jailed for ten years yesterday.

Adrian Ringland, 36, from Ilkeston, Derbyshire, used a spyware Trojan horse to infect and control the girls' PCs. He admitted posing as a teenage boy in internet chat rooms in order to plant the malware onto the user's computers. The victims believed they were opening a picture of their sham friend, but the attached file was infected and allowed Ringland to control their PCs.

The court heard how the father-of-two threatened to crash the teenager's systems unless they did what he asked. He reinforced his threats by switching their monitors on and off, moving their cursor around and opening and closing the disc drive. One victim described it as "like a scene from the Matrix".

Once the girls had emailed him one intimate photograph, he blackmailed them into sending more by threatening to forward the picture to people in their email address list.

He committed the crimes over three years and only stopped when he was caught in August 2005 after a Canadian girl reported him to the authorities. The charges also involved three girls in the UK, all aged between 13 and 14.

Ringland was jailed yesterday at Inner London Crown Court after he pleaded guilty to 20 charges connected to internet child abuse, two of indecency with a child, four of blackmail, four of hacking and ten offences of making indecent photographs of children.

One girl told him his depravities were "internet rape" and threatened suicide unless he left her alone. Lisa Wilding, prosecuting, told the court: "This is a case about manipulation, exploitation and sexual gratification and reveals the horrors that lurk within the internet and the minds of some individuals that use it."

In sentencing him, Judge Nicholas Philpot said, it was a "disgusting and abusive treatment of adolescent girls".

Graham Cluley, senior technology consultant at security firm Sophos, commented: "What's horrifying is it's only a matter of time before this happens again - these children will not be the last to be abused via the internet using spyware.

"It's encouraging to see the court take a hard line on this type of abuse, but it must be remembered that the internet gives paedophiles anonymity and access to children all around the world.

"It's essential that young people are taught how to behave safely online, to avoid being exploited by sick-minded hackers."