RUTH Kelly's future as the Westminster education secretary hung in the balance last night as more evidence emerged about how sex offenders are being allowed to work as teachers, and she moved to change vetting procedures.

In the change, it is believed her urgent policy review has decided to strip ministers of their power to judge whether sex offenders should be allowed to teach, and instead hand the decision to local experts.

However, notice was given from the Labour back benches that Ms Kelly, 37, now had just days to save her cabinet job after it was revealed a convicted child molester had been given the green light by the Department for Education to return to work as a maths teacher.

A leaked letter of January 2005 showed how the secretary of state, having "carefully considered all the information put before her" about William Gibson told him she had "decided not to bar or restrict your employment".

Mr Gibson, 59, was convicted of indecently assaulting a 15-year-old girl in 1980 and received a 30-month jail sentence for a string of offences. On Saturday, he was suspended from a boys' school in Bournemouth. School governors said he would not be returning.

The removal of Scottish ministers' role in decisions about sex offenders working with children is expected to be part of new legislation being put through Holyrood before the election next year. Scottish Executive ministers can add people to the list of those disqualified from working with children, with employers required to inform them if someone is sacked for inappropriate behaviour. However, there are only 63 people on that list, begun a year ago, and most of them were placed there as part of a court sentence.

Last week, the SNP highlighted the gulf between the 3025 people on the sex offenders' register at the start of last month, and the small number barred from working with children.

In response, Jack McConnell, the first minister, brought forward an announcement that next month it is to publish plans for a Scottish Central Barring Unit as a replacement for the disqualified list. A consultation will considerwhether it will be allowed to extend its reach to people placed on the sex offenders' register before the start of last year. A key feature of the new barring unit is to be continuous updating of information.