Just weeks before the tables are published, the National Association of Head Teachers is calling on the government to abandon a measure designed to track pupils' progress between the ages of 14 and 16.

It argues that this "statistically flawed" measure will penalise successful comprehensive and grammar schools.

The union's call for it to be dropped has won backing from the Conservatives.

David Hart: "The Secretary of State must withdraw this measure from the tables"

Teachers have demanded the introduction of "value added" information into the league tables since the process of publishing secondary school results began. They see this as a fairer way of measuring school performance than "raw" GCSE results, as it would show the improvement in pupils' achievements between the ages of 11 and 16.

But this year's partial value added measure will simply grade schools from A to E by comparing final year pupils' GCSE results with the national test results they achieved two years ago.

The General Secretary of the National Association of Head Teachers, David Hart, has written to the Education Secretary, David Blunkett, calling on him to abandon its inclusion in the tables.

In his letter, Mr Hart gives examples of schools which have already been told they will be placed in the below-average D category:

• A grammar school with 41% of its GCSE candidates achieving A* and A-grades.

• A grammar school with 100% of its GCSE candidates achieving A* to C grades in five or more subjects.

• A comprehensive school with 70% of its GCSE candidates achieving five or more A* to C grades.

"It is quite clear that the Department for Education and Employment proposes shortly to publish performance tables which will contain seriously flawed value added data," said Mr Hart.

"It is ironic that this will mislead the very people who are supposed to be enlightened by league tables.

"It will quite unjustifiably damage a significant number of schools which are successful by any reasonable criteria.

"The Department must go back to the drawing board and re-think the method by which the value added by schools to their students' education can be more properly assessed."

The Shadow Education Secretary, David Willetts, said: "Any value added measure must be very carefully designed to win widespread consent. This one clearly does not."

But the Schools Minister, Charles Clarke, said the government would not drop the new column from the tables.

"It will provide a much fairer measure of how schools are performing," he told BBC Radio 4's Today programme.