Much like a burglar robbing a house, cyber-criminals leave behind "fingerprints".

Kenneth Lockley posted child porn on the internet

It was these prints that led to the arrest of Kenneth Lockley in a cross-Atlantic sting operation between Scotland Yard and police in California.

The Derbyshire computer programmer was jailed this week for 18 months, the first person convicted in the UK for trying to incite an undercover officer to procure a child for sex over the internet.

Lockley, who worked for Core Design, a British subsidiary of the Lara Croft games giant Eidos, surfed the internet during office hours looking for underage sex.

He contacted a US site code-named Valerie's Escorts last April, run by the Los Angeles police to lure paedophiles.

Lockley worked for a British subsidery of Eidos

Using a false name, Lockley asked if there was an agency in the UK that could set him up with a child for sex.

"I need to have someone soon or I may rape a little girl instead and I do not really want to do that," he said.

Knowing only that Lockley lived in England, the Californian police contacted New Scotland Yard's paedophilia unit.

British detectives traced Lockley, and set up an undercover operation to trap him into believing he would meet a nine-year-old girl for sex at a London hotel.

'Not rocket science'

The police, reluctant to give away how they operate, would not discuss how they track cyber-paedophiles.

Colin Rose, whose company Actis Technology helps trace illegal activities online, says finding a repeat visitor to a known site is hardly rocket science.

The offender's computer commicates with other machines, and a record of this activity is logged.

"If you are browsing a site, you are being sent information - and that computer has to know where to send that information.

"There are ways to make yourself anonymous on the web, but any information to get to you has to go through certain sources."

His company is developing a system that will flag any illegal information downloaded on an office network.

"It will detect abuse of a corporate network, if hacking tools, information on drugs, or, most serious of all, paedophilia are being downloaded.

"We don't call anyone ourselves - we identify it and it is up to the company to take action."

Civil liberties

A proposed government bill that would give police more power to covertly watch online criminals could also close the net around paedophiles.

The Regulation of Investigatory Powers Bill gets its second reading in the House of Lords on 25 May.

The legislation has been heavily criticised for giving too much power to the police, and not doing enough to protect privacy and civil liberties.

The bill will force internet service providers to install links to an MI5 watching station that will be able to monitor the net activity of anyone they want.