Jonathan Collard, 48, who used to be head of geography at Great Walstead School, near Lindfield, West Sussex, was caught as part of Operation Ore, an investigation into the users of internet child pornography.

He admitted downloading more than 5,000 images and pleaded guilty to seven charges of making indecent photographs and 17 of possessing indecent photos.

Lewes Crown Court heard Collard, who lived at the school but later gave his address as Kirby Avenue, Ripon, North Yorkshire, had altered photographs he had taken of children to make indecent images.

'Evil and sordid'

Collard, who worked at the school since 1976, was arrested by police when they raided his caravan on the school grounds and found the pictures stored on his computer.

Defending him, Sarah Thorne said Collard had become obsessed with child porn and had difficulties forming relationships with adults.

Jailing him, Judge Richard Brown said his activities were "evil and sordid" and ordered him to sign the sex offenders' register for 10 years.

He also banned him from ever working with children and banned him from owning equipment which can access the internet.