Sex offenders' e-mail addresses are to be passed to social networking sites like Facebook and Bebo to prevent them contacting children.
Under government proposals, offenders who do not give police their address - or give a false one - would face up to five years in jail.
Websites would be expected to monitor the e-mail address usage or block them accessing the sites.
The Home Office said the new laws would apply to about 30,000 sex offenders.
Other measures in new government guidelines include a "kitemark" for filtering software.
Home Secretary Jacqui Smith said she wanted children to be "free from fear".
However, BBC home affairs correspondent Danny Shaw said ministers admitted that details of the system were still to be worked out, including how it would work with websites based abroad over which the UK has no jurisdiction.
Both Facebook and MySpace are based in California.
The new government guidance comes after the telecoms regulator Ofcom talked to 5,000 adults and 3,000 children and found nearly half of those aged between eight and 17 had a profile on social networking sites such as MySpace, Facebook and Bebo.
It also found 41% of the children surveyed had their profile set so anyone, rather than just friends, could view it.
We feel it will add protection for children using the internet
The recent Byron review also found about a third of those aged between nine and 19 who used the internet weekly had received sexual comments via e-mail, instant message, chat or text message.
Announcing the new guidelines, the home secretary said: "I want to see every child living their lives free from fear, whether they are meeting friends in a youth club or in a chat room.
"We are working together 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."
Shaun Kelly, from the children's charity NCH, told the BBC that he welcomed the new measures.
"We feel it will add protection for children using the internet.
"It will mean that those who have previously offended against children will be stopped from accessing certain websites and certain social networking sites that children and young people are known to use.
"I think that will increase children's online safety."
The Social Networking Guidance contains recommendations for service providers and safety advice for first-time users.
Arrangements for the industry and law enforcement agencies to share reports of potentially illegal activity and suspicious behaviour
Making it more difficult for people registered over the age of 18 to search for users under the age of 18
Encouraging children not to provide excessive information about themselves
Chief Executive of the Child Exploitation Online Protection Agency Jim Gamble said the guidance had the "real potential to accelerate online child protection".
"It will provide parents with those crucial indicators as to which sites and providers they should be using, allowing children the chance to get on and enjoy the full benefits of the internet with vital reassurance," he said.
|Liveleak on Facebook|