Paedophiles who adopt false identities to roam the internet in search of children to abuse face five years in jail under anti-stalking laws to be announced today.

Young computer users are being increasingly targeted by adults who pose as teenagers to strike up email 'friendships'. They may win trust over several weeks by exchanging harmless chat or sending attractive images to download before attempting to set up meetings or make sexual suggestions.

Home Secretary Jack Straw will outline detailed plans for 'paedophile prevention orders' - effectively injunctions in cyberspace which would target potential offenders before they reach the stage of physical contact.

Adults could provide enough grounds for police to seek an order through the magistrates' courts just by lying about their gender or age in online 'conversations' with children.

Breach of the order banning further contact would bring criminal proceedings, up to five years' jail and automatic inclusion on the sex offenders' register.

'These orders will mean that as soon as you feel there is a cause for concern you can get in before there is any contact,' said a Labour Party source. 'Such approaches are very difficult to tackle under existing laws.'

Critics will question how practical the orders will prove if offenders try to conceal their identities by using fake email accounts. Parents will still need to be vigilant to find out who their child is talking to online. Straw will call for software designed to protect children to be installed on all PCs targeting the family market.

His proposals stem from a taskforce set up by Ministers involving police, children's charities and internet pro-viders. Television presenter Carol Vorderman, who has campaigned on the issue, yesterday welcomed the move.

'I am delighted at the speed with which the Labour Party has responded and the pressure that this will put on the industry,' she said.