Education Secretary Ruth Kelly, who is facing fresh calls to resign in a row over sex offenders in schools, is to make a statement to MPs on Thursday.
Downing Street says she needs time to carry out a proper review of the number of registered sex offenders teaching in schools in England and Wales.
But shadow education secretary David Willetts said confidence in Ms Kelly was "ebbing away".
Meanwhile, a teacher at the centre of the row has denied being a paedophile.
Ms Kelly's review will look at all cases in the last 30 years.
Mr Willetts said Ms Kelly's speech to the Commons on Thursday was not soon enough.
"It's already a week since I asked Ruth Kelly a simple question how many people on the register of sex offenders did she clear to work in our schools. It's extraordinary she still needs longer to answer that basic question," he told BBC News.
William Gibson, 59, was cleared by the Education Secretary Ruth Kelly to teach despite his conviction for indecently assaulting a 15-year-old girl in 1980.
Mr Gibson, who had an affair with the girl, told the Bournemouth Echo: "I am not a risk to children."
He was suspended by the school after his past came to light. Ms Kelly has been under mounting pressure over the issue.
The Gibson case has added to the pressure on Ms Kelly since it emerged last week that one of her junior ministers had cleared a PE teacher to work in schools, despite a police caution for downloading child pornography.
William Gibson was checked and interviewed by the supply agency
The Department for Education is considering more changes to teacher recruitment in England and Wales after revelations that sex offenders were approved to teach.
It has already been agreed that future decisions will rest with professional experts, rather than politicians.
Downing Street says the prime minister continues to have confidence in Ms Kelly and she will stay in her job to push through school reforms.
It also denied the opposition's claims Ms Kelly had misled the Commons last week when she said the education department's teaching blacklist - List 99 - banned sex offenders from working with children for the rest of their life.
Tony Blair has been quoted as saying that it is better for Ms Kelly to take criticism at the moment, if she is then able to present a comprehensive review on the subject.
Former Chief Inspector of Schools, Chris Woodhead, said Ms Kelly should resign.
He told BBC Two's The Daily Politics show: "Every day, more evidence emerges of things that are unacceptable. Therefore, she should take responsibility."
Mr Gibson, convicted in 1980, was then employed by Portchester School in Bournemouth last September as a supply teacher despite having been removed from three other schools.
The agency that employed him - Step Teachers - said he was "suitable to work in schools" and that a supporting letter from one of Ms Kelly's officials had been "powerful".
In a statement, the firm's solicitor said it had at all times complied with the government's vetting procedure when recruiting teaching staff.
It also carried out an enhanced disclosure check with the Criminal Records Bureau which had brought to their attention "issues as to his [Mr Gibson's] suitability".
"These were investigated fully with such processes as an interview with Mr Gibson when he was asked to explain his position," the statement said.