Police investigations into users of child pornography websites could have major implications for the vetting of carers, the former president of the Association of Directors of Social Services warned.

Mike Leadbetter said the scale of the problem was so great that local authorities may have to carry out multiple checks on staff working with children far in excess of what is currently being done, and regularly quiz children about their treatment from carers.

Leadbetter, director of social services at Essex - a post he will be leaving at the end of December to become leader of the Department of Health's training task force - said recent investigations by US police had found there were more than 7,000 users of child porn websites from the UK.

As a result, police and local authorities have been trawling through the list of names - users were identified by paying to access websites with their credit cards - to try to track them down. The operation, codenamed Ore, has already resulted in some arrests.

Users of the websites have come from all backgrounds and a variety of occupations - including foster care.

Leadbetter said: "The implications of this are off the scale and could grind the child care system to a halt." He said there was a need for more standardisation in the way local authorities assess whether children are at risk of abuse.

"Different authorities have different interpretations of risk, while police deal with users in different ways," he said.