The UK government's new internet surveillance centre will become operational this summer, but it could make it harder rather than easier to catch paedophiles and terrorists by encouraging them to use stronger encryption.

The National Technical Assistance Centre (NTAC), based at MI5 headquarters, will decrypt seized computer data and intercepted internet and email traffic on behalf of law enforcement, security and intelligence agencies.

The Home Office said that without NTAC, technology developments will "lead to a considerable loss of intelligence from lawfully intercepted communications and evidence from lawfully seized material".

But NTAC is unlikely to have any impact on criminals using strong encryption on communications, according to Stephen Dyer, chairman of domain registrar centralNIC and member of the Internet Service Provider Association council.

"If you have an intercepted email and you don't have the key you can't decrypt it in the sort of time you need to prevent crime," he warned.

"The more overt technology you put in place to intercept traffic the more you encourage people to use encryption which is uncrackable," he added. "Paedophiles might then start using encryption which, to all intents and purposes, makes them invisible."

But Roland Perry, director of public policy at the London Internet Exchange, felt that this is overestimating most criminals' abilities.

"You can make assumptions about criminals being cleverer than they are," he said. "They still leave fingerprints behind even though they know that police can catch them that way."

And Cambridge University security researcher Ross Anderson maintained that, although modern encryption algorithms are theoretically unbreakable, the way they are implemented makes it possible to crack them.

"The cops can recover session keys and passwords from swap space, or install keyboard sniffers," he explained.

"And, even if the encryption can't be broken, most of the value that the police get from communications intelligence is from the traffic data - who is talking to whom - rather than from the content."

Separately, it was revealed last week that the FBI's Carnivore surveillance system had accidentally spied on innocent web users as part of an investigation into Osama bin Laden.

The FBI technician was so concerned that he destroyed all the emails.