Teachers' leaders condemned Hull and Knowsley councils' order as a "gross over-reaction" likely to make parents more concerned over their children's safety.
Leaders of the National Association of Schoolmasters Union of Women Teachers said the flood of requests could leave the Criminal Records Bureau unable to cope with demands to check new staff who have not yet been vetted.
Union leaders have been told that other councils may follow suit. They learnt of the checks just 24 hours after Ruth Kelly, the Secretary of State for Education, ordered a shake-up of the vetting system - and revealed that 88 convicted or suspected sex offenders had not been placed on List 99, the list that would automatically bar them from the profession. The majority, police told her department, are no longer involved in working with children.
Chris Keates, the general secretary of the NASUWT, said of the councils' move: "I cannot begin to quantify the distress such panic measures could cause to committed and dedicated staff and the burdens this will place on schools. The union is seeking legal advice over whether headteachers have to abide by the instruction."
She said, in both cases, it appeared the instruction had been delivered on the orders of the council's political leaders and that they smacked of "panic".
Knowsley Council's children's service said it had decided in October to carry out retrospective checks "on everyone ... including volunteers", at a cost of £100,000. "This decision was made in full consultation with the unions (including the NASUWT)," said a spokeswoman.
A spokeswoman for Hull City Council also denied it was "an over-reaction or a knee-jerk reaction". She said the council had already begun the process of checking all its staff who worked with children before the current furore.
It was disclosed yesterday that the Government took nearly two years to approve Paul Reeve to work as a PE teacher although he was on the sex offenders register. Ms Kelly said Norfolk police referred Mr Reeve's case to her department on 23 October 2003 - but it was 5 May 2005, the day of the general election, that ministers reached a decision.Reuse content