The Conservative leader, David Cameron, has called for the Education Secretary, Ruth Kelly, to be stripped of the leadership of the review of the handling of sex offenders in schools.
Mr Cameron said an independent person should be brought in to run the review, launched by Ms Kelly last week after teachers with cautions and convictions for child-sex offences were revealed to have been cleared to work in schools.
He stopped short of demanding Ms Kelly's resignation, but said it was wrong for ministers to be investigating their own decisions.
Mr Cameron's call came as Ms Kelly came under increasing pressure after her department was revealed to have given written permission for a man with a conviction for child-molesting to return to teaching. This is despite a claim made by the minister yesterday that British safeguards against allowing sex offenders near schoolchildren were "probably" the strictest in the world.
The Department for Education and Skills had told William Gibson, 59, that his actions had been "unwise", but said that the Secretary of State had decided "not to bar or restrict your employment".
Mr Gibson was suspended from work at the Portchester boys' school in Bournemouth this weekend after local authorities became aware of his conviction for indecently assaulting a girl aged 15 in 1980.
The maths teacher had been removed from three schools in the North-east and refused work by a supply agency which checked his details with the Criminal Records Bureau. But his details were never entered on the confidential List 99 maintained by the Department for Education of teachers barred from working in schools.
Mr Cameron said: "I think it is untenable for the inquiry to be led by ministers. Ministers are effectively going to be looking into the decisions that ministers have made. We need an independent person leading the inquiry so we get to the bottom of this."
Asked if Ms Kelly should be removed from her ministerial post, he said: "If it turns out that the decision-making was terrible and the system was poor, then clearly we will have to return to that question."
There was some relief for the Education Secretary when she faced party activists at Labour's national policy forum in Nottingham yesterday.
She told them that the measures taken in the UK to prevent paedophiles working in schools were "certainly the toughest in Europe, and probably the toughest in the world".
But members of the forum concentrated their fire on the Government's plans to change the way the country's 22,000 secondary schools decide on their pupil intake, which many fear will increase the gap between the best and worst schools.
"The delegates felt that the so-called paedophiles were a bit of a media story and a diversion from the real issue, which is the Education Bill," one said.
A letter, sent in January 2005 and signed by an official in the department, stated that Ms Kelly was aware of Mr Gibson's record. It said: "The Secretary of State has given weight to the fact that you accept that your actions were unwise and your behaviour was unacceptable; you understand the related consequences of your actions; and, you have undertaken teaching work in recent years to good effect."