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 base
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