Parents demanded an apology from British Education Secretary Ruth Kelly tonight after she admitted several registered sex offenders were allowed to work in schools.
Ms Kelly announced an urgent review after confirming Paul Reeve, who took up a job as a PE teacher, was not an isolated case.
With Tories calling for her to consider her position, Tony Blair issued an unprecedented vote of confidence, insisting she will keep her job.
But pressure on Ms Kelly mounted when Sir Michael Bichard, who headed the inquiry into the Soham killings, said he was "disappointed" the lessons of the tragedy had not been applied.
Mr Reeve was cleared to work at the Hewett School, in Norwich, last year, even though he received a police caution for accessing banned images of children on the internet. Around 10 other similar cases are thought to have been identified so far.
Margaret Morrissey, spokeswoman for the National Confederation of Parent Teacher Associations, called the news "terrifying".
"Ruth Kelly should apologise for allowing people on the sex offenders register to work in our schools," she said.
"She is going to have to look at her own conscience as a mother and politician over this issue."
Mr Reeve worked at the Hewitt School, Norwich, for eight days last month before police raised the alarm.
Although he was on the sex offenders' register, the Department for Education did not place him on the "List 99" of people barred from working in schools.
He was arrested and cautioned as part of Operation Ore, a major UK inquiry into child pornography, but not convicted.
Ministers decided the risks of allowing Mr Reeve to work at the school were "acceptable". It was thought tonight that the decision was taken by a junior minister rather than Ms Kelly despite suggestions at the weekend that she was responsible and stood by her actions.
The Education Secretary admitted there had been "a small number" of similar cases but refused to say how many until the review was complete.