Education Secretary Ruth Kelly is to rush through laws to tighten up restrictions on sex offenders working in schools in England.
The move follows the revelation ministers backed the appointment of a PE teacher even though he had a police caution for accessing child porn.
Ms Kelly says a "small number" of similar cases are now being reviewed.
The Tories say ministers have been too slow to put into force proposals made in the wake of the Soham murders.
The case was triggered by the case of PE teacher Paul Reeve, who was cleared to work at Hewett School in Norwich.
Mr Reeve was sent a letter by the head of the Safeguarding Children Unit on behalf of Ms Kelly.
It said she had taken into account testimonials suggesting he was "thought of as a trustworthy person" who would be a loss to the teaching profession, according to Channel 4 News.
Police caution: Given to people admitting minor offences - placed on sex offenders' register but not automatically barred from schools
Conviction: Defendant sentenced in court - goes on sex offenders register and put automatically on education blacklist, List 99
Ms Kelly told the Commons new laws proposed after the Soham murders would now be debated by MPs next month.
She said the government had "tightened significantly" List 99, the education department's blacklist, in recent years, making it more difficult for people of concern to get jobs in schools.
But she said it was impossible under current laws automatically to bar somebody who had accepted a caution for sex offences from working in schools.
Ms Kelly told MPs she would like cautions and convictions to be "treated identically", with the "closest possible alignment" between the sex offenders register and List 99.
'Livelihoods at stake'
But Downing Street said there were "complex legal issues" if the government went down that route.
"The question is should there be any discretionary element or should it be an automatic process?," the prime minister's official spokesman said.
There were also questions about the right to remove someone's livelihood and the government wanted to avoid a "knee-jerk" reaction, he added.
No 10 has already said Ms Kelly's job is not at risk.
Ms Kelly says the government will confirm the precise number - and whereabouts - of offenders working in schools and investigate "whether their behaviour has been of concern to the authorities".
And she will look at involving the police more closely in deciding who went on List 99.
She promised to make a Commons statement on the results of her investigation "as soon as possible".
Children's Commissioner Al Aynsley-Green said he was "profoundly concerned" by the revelations.
And Conservative shadow education secretary David Willetts said the case of PE teacher Paul Reeve at a Norwich school had "seriously undermined" public confidence.
Mr Willetts demanded to know how many people on the sex offenders register had been cleared by ministers to work in schools - and who had made the decision to clear Mr Reeve.
"You have hinted that it was your junior minister. He has replied that he is innocent. How can the department be in such disarray that it does not know who takes these decisions?," he asked Ms Kelly.
Mr Willetts said one of the main proposals from the Bichard inquiry, commissioned in the wake of the Soham murders, was that there should be a single list of sex offenders.
"It's extremely disappointing that all you can offer is another review and legislation which was in the government's own words 'urgent' 18 months ago," he said.
"Why did you fail to begin by setting out the basic principle that sex offenders should not be able to work in schools?"
Lib Dem education spokesman Ed Davey said Ms Kelly had two weeks "to answer the questions she failed to answer" in the Commons and "satisfy Parliament and parents that she is on top of the situation".
"The current system and lists are not fit for purpose. Parents will want to be confident that any review will be broad enough and tough enough to sort this mess out," he said.
But Barry Sheerman, Labour chairman of the education select committee, said decisions should be taken calmly and not amid a media-driven "hysteria".