Ruth Kelly, the Education Secretary, said today that she accepted full responsibility for the "extraordinary" ministerial lapse which allowed around ten registered sex offenders to work in schools.

The Education Secretary said that she had commissioned an "exhaustive" review into every case where adults on the Sex Offenders Register had been kept off the Government's teaching blacklist, known as List 99.

With the controversy threatening to build into crisis, Downing Street issued a statement this afternoon insisting that Ms Kelly's position was not under threat.

In a brief televised statement this evening, Ms Kelly told reporters that she took "full responsibility" for decisions taken on whether sex offenders were placed on List 99.

Ms Kelly repeatedly refused to state how many registered offenders had been cleared to work with children, saying only that initial investigations suggested "only a small number".

Asked whether the list would be taken out of the hands of "amateurs" she said simply: "These are really serious issues for child protection and the Government. Child protection is our number one priority, we have got to get it right.

"I have commissioned an exhaustive review and we will take immediate steps to tighten up the system. We will make sure as closely as we can do that anyone placed on the Sex Offenders Register is barred from working with children for life.

"This case involves very complex issues and I want to make it clear that I accept full responsibility for everything that has been done."

In a statement earlier today, the embattled Education Secretary had sought to distance herself from the decision to clear Paul Reeve, a Norfolk man who had received a police caution for accessing child porn online, to work as a PE teacher.

She said: "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."

Sources at the Department for Education and Skills say that the decision to clear Mr Reeve to work in schools was taken by a junior minister on duty at the department, although officials will not say which one.

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.

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