Thousands of staff have been sent letters telling them they are not eligible for the scheme because it contravenes teachers' pay and conditions regulations.
One secondary teacher, who had her vouchers withdrawn this month, said:
"This flies in the face of the Government's claims to be helping key workers with child- care and other expenses."
She said the scheme had saved her a modest but important sum each month, and she had negotiated her hours on the basis of changes to her salary and that her child would be looked after.
Ministers are now under pressure to change the statutory pay arrangements.
The Government has made great play of its plans for childcare expansion, particularly in extended schools. Under the childcare scheme, parents can agree to be paid less in return for a childcare voucher. Their benefits come in tax and national insurance exemptions on the sacrificed portion of salary.
But lawyers acting for the Department for Education and Skills said it fell outside the pay framework and was therefore unlawful.
Oxfordshire county council sent out a letter to its teachers informing them they were ineligible. "Unfortunately it is out of the council's jurisdiction," it said.
Andy Innet, of the National Employers' Organisation for School Teachers, said: "We have had preliminary talks with the Department for Education and Skills and hope we can come to a sensible solution."
A spokeswoman for the National Union of Teachers said: "It was clearly an oversight."
A DfES spokesman said: "The terms and conditions of the school teachers'
pay and conditions document make no provision for salary sacrifice."
"Teachers may, however, be paid recruitment and retention incentives and benefits of any sort and of any value, so could be given childcare vouchers as an additional pay incentive."
"Whether amendments to the document should be made is a matter which the department is considering and will discuss with its pay partners."