Education Secretary Ruth Kelly has announced an urgent review of the systems by which people are allowed to work with children.
Ms Kelly has been criticised for clearing the teacher for work
Her department cleared a man to work as a teacher despite his name being on the sex offenders' register after a police caution for viewing child pornography.
Others on the register were not on a separate proscribed list, she admitted.
Parents' groups have demanded an apology but Downing Street dismissed reports the minister's job was at risk.
A spokesman for Tony Blair said the PM had full confidence in Ms Kelly.
It is thought a junior minister took the decision not to place Paul Reeve - who was given a job as a PE teacher at the Hewett School in Norwich last month on List 99, which bans people from working in schools in England.
But Norfolk Police had reportedly followed strict Home Office guidelines in placing the teacher on the sex offenders' register and he resigned from his post after they intervened.
Ms Kelly said in a written statement to MPs she took "full responsibility" for decisions taken in the department on whether individuals should be placed on List 99.
Speaking later to reporters, she said the issues involved were "really serious" for child protection and for the government.
There had been mounting concern that registered sex offenders might not be placed automatically on her department's banned list.
It is understood about 10 other similar cases have been identified.
"These are complex issues taken on the basis of professional advice including police advice," Ms Kelly said.
Sir Michael Bichard, meanwhile, said he was "disappointed" the vetting recommendations of his inquiry into the Soham murders had not been acted on.
Ian Huntley was able to get a job as a caretaker at a college on the same grounds as his victims' primary school despite a string of sex allegations against him.
"I was suggesting one barring or registration scheme," Sir Michael told BBC2's Newsnight programme.
"The problem is, as I understand it, that it's not been possible to find a slot in the legislative timetable for the necessary bill."
The DfES wrote to Norfolk County Council in May last year to say it had approved Mr Reeve to work in schools.
It said "the risks of the teacher being allowed to continue teaching were acceptable".
Ms Kelly said her review would make public all such decisions since the sex offenders' register was created.
It would cover how the "closest possible alignment" between the sex offenders' register and List 99 could be achieved and how police could be more fully consulted before decisions on including individuals on List 99 were reached.
One aspect of the review would be to consider whether ministers ought to be making such decisions at all.
Shadow education secretary David Willetts said her admission of other cases in which people on the sex offenders' register had not been banned from working in schools was "extraordinary".
The review will look at aligning the two lists
"Confidence in the Secretary of State is rapidly disappearing," Mr Willetts said.
His concerns were echoed by Liberal Democrat spokesman Edward Davey.
"This serious lapse of judgement by Ruth Kelly has exposed an extremely worrying state of affairs," he said.
"Many people will be shocked that this type of decision is taken by a minister and not an expert group."
Margaret Morrissey, from the National Confederation of Parent Teacher Associations, described the case as "terrifying".
"Ruth Kelly should apologise for allowing people on the sex offenders register to work in our schools," she said.