The six-term school year will be introduced from September 2004 despite opposition from councillors and unions.

Labour and Liberal Democrat members on Oxfordshire County Council have called for a re-think on the decision to introduce a new school calendar. A teaching union has said that plans will create "self-inflicted chaos" unless the rest of the country falls in line.

Members of the council's learning and culture scrutiny committee said there was not enough time to plan for the change before next year, the planned-for summer holidays were not long enough, and consultation had been inadequate.

Labour member Andrew Brown said: "We are not saying that a six-term year is the wrong way to go, but the speed with which the decision was taken would have made things very difficult, particularly for teachers."

Members of the NASUWT (National Association of Schoolmasters Union of Women Teachers) called on the county council to pull out of the plans and focus their efforts on persuading the rest of England and Wales to join them.

The union has already encouraged 10 councils to drop the scheme until all counties are involved, but 19 authorities have pressed on with the conversion.

Oxfordshire County Council says the benefits outweigh the costs.

Geoff Branner, secretary of Oxfordshire NASUWT said: "If Oxfordshire now goes ahead, it will create huge difficulties for everyone who works in Oxfordshire schools who has children in neighbouring counties' schools. The last thing Oxfordshire education needs just now is a bout of self-inflicted chaos."

The union agreed with the principle of a six-term year provided it was introduced simultaneously across the country.

The 19 authorities which have adopted the plan include Gloucestershire and Wiltshire, but Buckinghamshire, Warwickshire, Northamptonshire and Reading have not.