Ministers have cleared more adults on the sex offenders register to work in schools, the Education Secretary Ruth Kelly confirmed today.
Ms Kelly announced an urgent review of the situation as she faced continued pressure over the case of PE teacher Paul Reeve.
Mr Reeve was given a job at the Hewett School in Norwich last month despite being cautioned for accessing banned images of children on the internet.
Ministers at the Department for Education and Skills decided last year that the risk of allowing Mr Reeve to work at the school were "acceptable".
Sources suggested that the decision was approved by a junior minister on duty at the DfES rather than Ms Kelly personally.
But in a statement issued to MPs, the Education Secretary promised she would consider any similar cases in person in the future.
Ms Kelly said: "The protection of children is a top priority for this Government.
"The case which has come to public attention this week raises very serious issues.
"I take full responsibility for all decisions taken in the department on whether individuals should be placed on List 99, the list covering those barred from working in the schools.
"Initial inquiries indicate that there have been a small number of cases where ministers have made the decision not to include on List 99 an individual on the sex offenders register.
"It is of the utmost importance that the House and wider public are not misled by any information put into the public domain.
"I have therefore decided to commission, as a matter of urgency, an exhaustive review of all such cases since the introduction of the sex offenders register, the decision-making process surrounding such cases and any immediate policy implications."
She continued: "In the meantime, with immediate effect, I will personally consider any cases of individuals on the sex offenders register where a ministerial decision is required as to whether they should be placed on List 99."
Mr Reeve was arrested in 2003 by Norfolk Police as part of Operation Ore, the largest inquiry into child pornography undertaken in the UK.
He received a police caution for accessing banned images of children on the internet.
But he was only stopped from working as a PE teacher when police, who had warned he was a risk, alerted the school's headteacher.
Ministers are thought to have considered evidence that he had accessed paedophile websites inconclusive so did not place him on List 99.
The Department for Education wrote to Norfolk County Council saying that the Government had "considered all aspects of the case, including sex offender registration, and decided that the risks of the teacher being allowed to continue teaching were acceptable".
Parents' groups expressed "shock" at the decision and teachers' leaders said no one on the sex offenders register should ever have been given work in a school.
The Conservatives accused Ms Kelly of a "serious lapse of judgment" .
Ms Kelly said she would consider as part of the review how to achieve " the closest possible alignment" between List 99, which is kept at the DfES, and the sex offenders register.
She also promised to examine the role of ministers in the process of deciding who should be barred from working in schools.
The review will consider how advice from the police can play a bigger role in deciding which individuals are placed on List 99.
Ms Kelly said the Government was already committed to implementing Sir Michael Bichard's recommendations on tightening the vetting system for adults who want to work with children contained in his report into the Soham tragedy.
The Shadow education secretary David Willetts said the news that there have been other such cases was "extraordinary".
"We now need to know as a matter of urgency how many such cases there have been, and whether heads were made aware they were employing someone on the sex offenders register.
"I asked the Secretary of State on Monday about her personal involvement in these decisions.
"It seems from her statement that she is now trying to shift responsibility to others by saying that from now on she will consider cases personally.
"We still need to know if she personally took the decision about Mr Reeve, or if not, which minister or official was acting in her name.
"The more this affair drags on, the more serious are the concerns about the Education Secretary's judgment.
"We still need straight answers to the questions I put to her on Monday, instead of more ducking and diving.
"This is not a matter of party politics. It will concern every parent across the country."
Steve Sinnott, general secretary of the National Union of Teachers, said: "Children should not be put at risk. That should be the guiding principle.
"It is quite astonishing that the need for a link between the sex offenders register and List 99 does not appear to have been properly considered in the past.
"Of course, there needs to be protection for teachers who may mistakenly have their name sent to the DfES for inclusion on List 99.
"But to decide not to include someone without good reason who has been included on the sex offenders register leaves children at risk. That is unacceptable."Reuse content