The education secretary has commissioned an "exhaustive review" into cases of sex offenders being allowed to work with children.

Ruth Kelly is facing mounting criticism over the decision by her department to allow a registered sex offender to work as a PE teacher.

A review is under way into the case of Paul Reeve, who had been cautioned for accessing banned images of children, and the possibility that there may have been other mistakes made by ministers and officials.

Admitting that she could not say how many sex offenders were working with children, the education secretary insisted she would act to make children safe.

"Protecting children is our number one priority. We have got to get it right and it is vital for public confidence in the system," Kelly told reporters on Wednesday night.

"It is for that reason and because of the case that was raised this week that I have commissioned an exhaustive review of all the cases and decisions that have been made since the sex offenders register was introduced in 1997."

As she faces mounting political pressure Kelly said she had taken urgent action to ensure that a full assessment of the problem is made.

"I have also announced immediate steps that I am going to consider to tighten up the system, including for example, trying to make sure as closely as we possibly can, that everyone who is placed on the sex offenders register is also barred from working with children for life," she said.

"And also whether we can take police advice and incorporate that more fully in the decision making process.

"I also want to make it absolutely clear that I accept full responsibility for every decision that has been made in the department."

Kelly had earlier issued a written statement to MPs in which she said that there may also be a "small number of cases" where sex offenders have not been banned from working in schools.

Pressure mounts

However claims that she is about to lose her job were dismissed by Downing Street.  "A number of reports suggesting her job was in jeopardy are simply wrong," the prime minister's official spokesman said.

But the government's political opponents are sharpening their knives as the possible crisis escalates.

Shadow education secretary David Willetts said: "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.

"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.

"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."

Liberal Democrat spokesman Edward Davey agreed that: "This serious lapse of judgement by Ruth Kelly has exposed an extremely worrying state of affairs.

"Many people will be shocked that this type of decision is taken by a minister and not an expert group. Parents will want to know there is going to be better collaboration between government departments in future."