Ruth Kelly, the Education Secretary, insisted tonight that protecting children was her "first priority" after she was criticised by parents and teachers for allowing a man on the sex offenders’ register to work in a school.

Ms Kelly promised to make "further improvements" to the policies under which she approved Paul Reeve for work as a PE teacher if reforms were found to be necessary.

But in her first public comments on the case, Ms Kelly did not explain why she had taken her decision, saying that she could not discuss individual cases. 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.

Parents’ leaders described the episode as "terrifying" and the Tories accused Ms Kelly of "a serious lapse of judgment" after details of the case emerged at the weekend.

Ms Kelly said in a statement tonight: "Our first priority has always been the protection of children. Whilst we do not comment on individual cases, decisions have to be taken on the basis of the evidence and within the legal framework.

"We are already reforming the current system by developing a new vetting and barring scheme with the Home Office and police which will provide better protection for children and vulnerable adults.

"I have also asked my officials to review this case to see whether any further changes are necessary. We will consider for example whether we can ensure greater correlation between the sex offenders’ register and List 99. If further improvements are needed on the basis of advice received, I won’t hesitate in making them."

The National Confederation of Parent Teacher Associations had earlier called on Ms Kelly to explain why she took the decision to allow Mr Reeve to work as a PE teacher. Margaret Morrissey, NCPTA spokeswoman, said: "Parents will be really shocked and will expect the Secretary of State to explain to them why this has been allowed to happen.

"It is absolutely terrifying. Our big concern is that if there is a problem with the system, which there evidently is, then how many other people are slipping through the loophole? Parents are going to need some very swift information and reassurances about what is going on."

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 Mr Reeve, believed to be in his 30s, was only stopped from working as a PE teacher when police, who had warned he was a risk, alerted the school’s headteacher.

Ms Kelly considered evidence that he had accessed paedophile websites inconclusive and so did not place him on "List 99", the list of people barred from working in schools. Teachers’ leaders said, however, that the fact someone is on the sex offenders’ register should be enough to ban them from working in schools.

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