The IT industry has been urged to build spam filters that work into computers and to make them available for free, if the internet is to remain open to all.

John Carr, internet consultant to the NCH Action for Children, warned that it is unrealistic for companies to expect parents to monitor their children's online activity, and called on industry to improve filtering techniques.

Speaking at the UK Spam Summit, held in Parliament last week, he told delegates that if action is not taken soon, there will be two internets - a walled garden of acceptable sites and one where any sort of material can be seen.

The charity is concerned that as more children sign up for email accounts, they will receive spam where clicking on a link exposes them to websites peddling sex and scams.

But he maintained that any solution to block spam should be free, because parents do not believe they should have to pay for it to be stopped.

"There should be some safety measures built into computers sold into the domestic market," said Carr.

The argument that parents should closely monitor their children's online activity "does not recognise the reality of modern parenting", he added.

He also pointed out that, with 3G phones on the horizon, it will not be long before children have the internet in their hands wherever they are.

But Richard Allan, MP for Sheffield Hallam, argued that there would always be a trade-off between security and cost.

"One of the reasons email is so cheap is because it's relatively insecure," he said.

"The idea that we can have a more secure internet without paying for it has got to be kicked off the agenda."