Thousands who view child abuse images online should be treated as patients by the NHS rather than sent to prison because they pose no threat to children, says one of Britain’s leading police officers.

In an interview with the Guardian, Simon Bailey, chief constable of Norfolk police and the Association of Chief Police Officers’ (Acpo) lead on child protection and abuse investigations, said that while police had a database of 50,000 people who regularly viewed indecent images of children, research suggested not all were an immediate threat.

“What academic research would say is between 16% and 50% of those people who have viewed indecent images of children are then likely to be ‘contact abusers’ [of children]. That can be as high as 25,000 or as low as 8,000. [This group] poses a threat,” he said. However, the remaining group of child sex offenders – who are committing a crime by viewing the material online – are “non-contact abusers” who Bailey says do not “need to come into the criminal justice system in terms of being put forward before a court”.

He added: “We have to think about an alternative solution. [We] need to engage with service providers from mental health and the health service to work with us to say these people need help.”

The new approach provoked a debate among child protection experts and health professionals over whether the police were in effect decriminalising child sex offences at a time when online abuse appears to be increasing.

David Cameron will call next week for further controls over child abuse images. There are thought to be more than 100m of them in circulation on the web, up from 7,000 in 1990.

Admitting the new strategy appeared “a very unpalatable response from a senior police officer,” Bailey said the decision to give priority to active paedophiles rather than browsers of images that include the rape and torture of children was “based on realism ... it is based upon the fact there will be a significant number of those people who will simply not go on to contact abuse.”

This week a doctor, Myles Bradbury, was jailed for 22 years for “grotesque” abuse of 18 vulnerable children in his care. Police in Canada had told their British counterparts he was buying indecent images of children on the internet, but UK investigators failed to act for 14 months, classifying him as low risk.

The new strategy, which is being implemented by police forces and Ceop, the child exploitation and online protection command of the National Crime Agency, is a reversal of official thinking which has until now been that “anyone who possesses [images] poses a risk of committing contact sexual offences against children”.

Some have welcomed the shift. Jackie Craissati, fellow of the British Psychological Society and clinical and forensic director at an NHS trust, told the Guardian her reading of recent studies suggests 25% to 33% of web users of child abuse imagery should be “worried about”. She said: “A much larger group have nothing to suggest they are unconnected sex offenders.” Craissati welcomed the move to seek help from mental health specialists, but warned it would not be easy to get the NHS to engage with child sex offenders.

“First, if they had children, social services would have something to say about whether their kids could stay with them. Second, we are a long way off the NHS being comfortable about saying it is the health service’s job to treat these people. Third, the therapy is only effective for serious deviance. If you aren’t a problem then it does not work.”

Others were sharply critical. Jim Gamble, a former head of Ceop and now safeguarding chair for City of London and Hackney, said there had always been an academic debate about what proportion of men who download explicit sexual images of children also molest them.

He said it was conclusively answered by a 2008 US study of convicted web child sex abuse offenders which revealed 85% had admitted when taking lie detector tests that they had committed previously unacknowledged acts of sexual abuse against children, including rape.

“This is a gimmick by the police. It’s a nonsense. [Paedophiles] are manipulative and lie. They try to convince everyone else they are normal. But they are not. They are upstairs in their bedrooms looking at pictures of child rape. The youngest child in these images I saw when I ran Ceop had an umbilical cord. I would ask these academics who think people who are viewing this stuff are OK if they would let these guys babysit their daughter or grandchildren.”

Bailey rebutted the idea that the police had “gone soft” on child sex crime, pointing out that in the 12 months since October last year 715 people had been arrested for possession of indecent images – the largest single haul in Ceop history. Last year fewer than 200 people were arrested.

He said: “We are at the beginning of a journey. That is why [Ceop boss] Johnny Gwynn and I are driving this agenda and starting to tackle the threat posed. The figures bear it out.”