The government has apparently announced a change of policy on the employment of teachers in England's schools in response to alarm over sex offenders.
Education Secretary Ruth Kelly has promised to tighten the system.
The Department for Education and Skills has said a criminal records check must be carried out on someone before they start work.
But until now the guidance has been that provided someone was not on a banned list, they could begin work while the criminal records check was carried out - which can take weeks.
It has also been revealed that people on the list - List 99 - have had the prohibition relaxed to continue working in schools.
The Times newspaper highlighted cases in which teachers on List 99 have, on appeal, been allowed to work with different types of children to those that had given rise to concerns about their behaviour.
The policy change on recruitment procedures followed earlier renewed concerns about loopholes in the list.
It emerged that a man with a conviction for indecently assaulting a child in 1980, and more recent convictions for fraud, forgery and theft, had obtained work in schools in the North East because he was not on List 99.
On three occasions he was dismissed once the Criminal Records Bureau (CRB) check - known as a disclosure - revealed his record.
The department in a statement reiterated that all employers - schools and supply agencies - must carry out "the necessary checks" which would reveal whether someone had a conviction.
It added: "Employers should obtain a CRB Enhanced Disclosure in respect of all teachers they recruit before the person is placed in a school."
But that is not what the existing guidance said.
List 99 discretion
To manage the chronic problems in the delivery of disclosures which followed the setting up of the CRB, the government said in 2002 that teachers and teaching assistants could be put into schools on the basis only of the List 99 checks.
The system was tightened after Ian Huntley murdered two schoolgirls.
Following the murders of 10-year-olds Jessica Chapman and Holly Wells by a school caretaker, it scrapped this and insisted all new staff must obtain a full disclosure from the CRB before starting work.
But the CRB failed to cope and schools had to turn children away because they did not have enough fully-vetted staff.
So the then education secretary, Estelle Morris, gave head teachers discretion to let people start work after List 99 checks, with a full disclosure to follow later.
That has remained the situation.
As of Saturday the CRB website was still telling people that "the Department for Education and Skills allows applicants, who have applied for a CRB check, to start work in a school under supervision if they are not on List 99".
In the latest case, a 59-year-old maths teacher called William Gibson obtained temporary work in schools in South Tyneside and Co Durham before his past was uncovered.
South Tyneside Council said he had been employed on the basis of initial checks, but was "deemed unsuitable to work with children once enhanced checks had been processed".
One long-standing loophole with List 99 is that people are only on it if their suitability to work with children became an issue while they were already doing so.
So if someone changed careers from financial adviser to teacher, past wrongdoing would not have put them on List 99.
Another loophole was exposed earlier in the week with the case of a Norfolk PE teacher, Paul Reeve.
He had applied for a job despite having a police caution for accessing child pornography.
It emerged that a junior minister, Kim Howells, had used the education department's discretion to say he need not be put on List 99.
The Education Secretary, Ruth Kelly, then admitted there were other similar cases and promised to tighten up the system.