The speed with which websites can spring up and move can also catch out some filtering systems, said Mr Haggart.

BBC: The problems of finding filth

But, said Mr Haggart, many filtering systems are too crude and let through sites they should block and stop those that are innocent.

BBC: The problems of finding filth

"There's no guarantee that they are going to be able to get the offensive material out of the system quickly, " said Geoff Haggart, European head of filtering firm WebSense.

BBC: The problems of finding filth