A defiant Education Secretary insisted today that Britain had "some of the toughest child protection laws in Europe".
But Ruth Kelly was forced to admit to MPs that she did not know the " precise number" of cases where people on the sex offenders register had been allowed to take up teaching jobs.
She told the Commons that finding out the number was one of the purposes of an "exhaustive review" she has set up in the wake of the case of Paul Reeve.
He was cleared to work at the Hewett School, Norwich, last year, even though he received a police caution for accessing banned images of children on the internet.
Ms Kelly said the inquiry would look into the whereabouts of sex offenders working in schools and whether their behaviour had caused concern.
She explained that where people had been cautioned for a sex offence but not convicted by the courts, "the law currently requires each case to be considered individually and a decision taken by ministers based on evidence and advice, even though the individual may have been placed by the police on the sex offenders register".
She went on: "Initial inquiries indicated that there have been a small number of such difficult cases. I fully understand the concern this has caused and I'm determined to do something about it."
She said she had ordered "an exhaustive review of all such cases in order to confirm the precise number of these individuals, their whereabouts and whether their behaviour has been of concern to the authorities.
"These cases raise questions about whether these long-standing arrangements need to be changed."
Earlier Downing Street insisted Ms Kelly was secure in her post, despite calls from the Opposition for her to consider resigning.
Tony Blair's official spokesman said: "Ruth Kelly's position remains unchanged and will remain unchanged."Reuse content