The discovery plunges the whole system of safeguarding children from sex offenders into renewed doubt and is likely to put further pressure on Ruth Kelly, the Education Secretary. Despite the Education Department insisting that the so-called List 99 was an “absolute bar” on sex offenders working in schools, The Times has established that men convicted of child molesting and pornography offences have remained free to teach. Teaching unions and opposition politicians expressed astonishment last night and demanded an explanation from Ms Kelly.

In one case, a man convicted of possessing pornographic pictures of young boys masturbating was given permission by the Education Secretary to teach, provided he taught only in girls’ schools.

Another man who acknowledged that he was a “homosexual paedophile” with an interest in young boys, and who had indecently assaulted a child, was allowed to continue teaching as long as he taught GCSE subjects only.

A third man succeeded in overturning a total ban so that he could teach in boarding schools as long as he did not reside on the premises. The man had written “inappropriate words” about a pupil at his preparatory school.

The discovery provides a fresh twist to the problems faced by Ms Kelly. Until now, controversy has centred on the procedures for deciding whether somebody should be placed on List 99. Now it has emerged that even when teachers have been placed on the blacklist, they can get the terms of the ban relaxed to allow them to work with particular types of children or in specific categories of school.

Teachers on List 99 can appeal to the Care Standards Tribunal, a body established in 1999 under the Protection of Children Act. It hears cases involving registers of childminders, social workers and other social services staff and healthcare agencies.

It also hears appeals against decisions by the Education Secretary to restrict or prohibit people from teaching or working in schools. The Times has found examples from the past five years of individuals with paedophile backgrounds who have been granted permission to work in schools.

The cases have emerged only because details appear on a website of the tribunal’s published decisions. It is unknown how many of the 15,000 names on List 99 may have limited permission to work with some types of children or certain kinds of school.

Officials at the DfES appeared to be unaware of the loophole, insisting that people on List 99 faced an “automatic bar” on working with children. “The whole issue that we are considering is about those cases where there is discretion about putting someone on List 99. Once you are on List 99, you can’t work in schools,” a spokesman said.

David Willetts, the Shadow Education Secretary, demanded to know why Ms Kelly had said nothing about the apparent shortcomings in List 99 in her two statements to the Commons.

John Dunford, the general secretary of the Association of School and College Leaders, said: “We have to make it absolutely clear that people on List 99 are not fit to be teachers, full stop.”

A spokeswoman for the National Union of Teachers said: “The union is astonished that anybody who has sexually abused a child or been cautioned for such involvement, should be allowed to teach children over 14 but not under 14. What protection does that offer the 14 year old?” It was Estelle Morris, then Education Secretary, who decided in August 2001 to allow Keith Hudson, the man convicted of possessing pornographic pictures of boys, to continue to teach “in an establishment which does not admit male pupils”. He sought to challenge even that restriction, but the tribunal upheld the Education Secretary’s original decision, saying that the risk of further offending was low.

The decision to let a male paedophile teach girls was made in spite of DfES officials recognising that child molesters can switch between the sexes. In evidence to the tribunal, Anne Hunter, of the Teachers’ Misconduct Team, said that 21 per cent of paedophiles interested in prepubescent children offend with either sex.

Hudson told The Times yesterday that he not yet applied for jobs in girls’ schools because there were “very few” in the area. He insisted that his ban had been unfair. “I should never have been convicted in the first place.”

He said that he had been contemplating appealing to Ms Kelly “but it doesn’t seem such a good idea now”.

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