England's senior judge said unconvicted paedophiles who were considered dangerous could be detained under a new type of "civil detention".

His remarks took lawyers, civil liberties bodies and government departments by surprise. Lord Woolf has been regarded by successive governments as an arch liberal and has often warned ministers against trampling on individual rights.

A spokesman for the lord chancellor's department said: "He's independent; he can say what he likes." The Home Office said the government was considering longer sentences and indeterminate sentences for some sex offenders but the idea of protective custody was not "under active consideration by us".

Lord Woolf's comments was seen yesterday as an encouragement to the government to consider extending to paedophiles its controversial plans for the detention of dangerous people with personality disorders who have committed no crime.

The plans, outlined a year ago in a white paper, would reform mental health legislation which allows compulsory detention only for treatable mental illnesses. Personality disorders are widely regarded as untreatable; paedophilia is notoriously difficult to treat.

John Wadham, director of the civil rights group Liberty, said: "I'm very surprised that the lord chief justice, whose role is to protect and safeguard our fundamental rights, should be suggesting provisions that will undermine and damage those rights."

He said the European convention on human rights would allow paedophiles to be detained against their will only if they were of unsound mind. "You can't detain somebody just because he's a paedophile."

Lord Woolf made his remarks in an interview on Radio 4's Today programme, in which he floated ideas for the ways in which the criminal justice system might respond to predatory paedophiles like Roy Whiting, convicted two weeks ago of murdering eight-year-old Sarah Payne. Whiting had served an earlier jail term for abducting and indecently assaulting another child, who survived. He had refused to undergo treatment.

Lord Woolf said: "It may well be, and this is a matter of very great sensitivity, that we have got to think, for those who are persistent offenders, of having some form of protective custody."

The law already allowed for a serious offender, considered to pose a significant risk of reoffending, to receive a discretionary life sentence.

But it might be necessary for parliament to consider further alternatives. "There may be a form of civil detention without having to prove a person has committed an actual crime."

It might be that "we would have to think about coming to the conclusion that there are a small, a very small minority of people, in the community against whom the public are entitled to be protected."

He added: "The problem is that if you have got an illness which can be treated, then you can be sent to a hospital com pulsorily because of the illness. But you have got to have an illness which is treatable for that purpose.

"We can't do that often with paedophiles because the condition is not treatable.

"And it may be, although this would be a huge infringement on the individual's rights, but we have got to think of the rights of those who would be offended against as well, if a case is made out that this person is a danger, that we would have to have some form of protective custody."

Paul Cavadino, director of policy at Nacro, the National Association for the Care and Resettlement of Offenders, said it would be extremely dif ficult to assess the threat posed by an individual who had yet to commit an offence. "I can't see how it is realistically possible to judge that somebody is dangerous unless they have acted in a way that shows they are dangerous. In practice, that means that they have either committed an offence or attempted to do so."