The Home Office will unveil plans today to bolster protection for children online, including a requirement for sex offenders to hand over their email addresses to the police.

The new legislation, which will come into effect later this year, applies to more than 30,000 people registered on the sex offenders' list.

Offenders who fail to give up their email address to police, or handover a false one, face up to five years in jail.

Paedophiles' email addresses will also be passed to social networking sites such as Facebook, Bebo and MySpace to prevent them preying on children.

"We have some of the strictest controls on sex offenders in the world to protect our children," said Home Secretary Jacqui Smith.

"We are working with police, industry and charities to create a hostile environment for sex offenders on the internet, and are determined to make it as hard for predators to strike online as in the real world."

However, the announcement has drawn criticism from some parent bodies which have pointed out that new email accounts and online profiles can be created in a matter of minutes.

"This is wonderful idea, but totally unworkable," said Andrew Hibberd, director of The Parent Organisation.

"Even if ISPs cut off their home broadband connections, there are absolutely no controls in place to stop [sex offenders] using a computer elsewhere or creating a number of email addresses using false details."

The Home Office move comes amid growing concerns that sex offenders are using social networking sites to groom youngsters.

A recent Ofcom study found that more than a quarter of eight to 11 year-olds with internet access claim to have a profile on a social networking website, despite the existence of pre-teen restrictions.