The National Association of Head Teachers (NAHT) claims that the tables, due to be published for the first time on 4 March, do not represent schools' real performance because they are based on the number of 11-year-olds eligible to sit the tests, not the number who actually sit them.
That means that a school where a high number of pupils missed the maths, English and science tests last summer through sickness or other reasons would come low down on the tables, even if those who sat the tests scored highly.
The union, which is represented by Cherie Blair QC and is using a primary school near Brentford, Essex, as a test case, wants the tables stopped unless the scores are changed to take absences into account. It also says the results of children with special needs studying in mainstream schools should be left out.
The Secretary of State for Education and Employment, Gill-ian Shephard, last night dismissed the NAHT's action as "nothing short of outrageous", claiming it would deny parents information they were entitled to.
The latest battle comes after 12 months of controversy surrounding the tables. A year ago, Mrs Shephard assured teachers there was no intention of publishing league tables of results until the tests for 11-year-olds had fully "bedded down".
Just weeks afterwards, she went back on her pledge under right-wing pressure and announced publication of the tables this spring, provoking a storm of criticism from teaching unions and threats by the NAHT to boycott the tests.
Earlier this month, Labour's education spokesman, David Blunkett, hinted that he was keen to scrap national primary tables in favour of local versions produced by local authorities. He later insisted the party was sticking by its promise to judge the usefulness of the first set of tables, but Mrs Shephard seized on the reports as evidence of Labour's intention to withhold information on school standards.
The NAHT's legal case centres on West Horndon Primary School, where 17 pupils on a roll of 120 were eligible to take the tests last summer. In the event, four were absent, dragging down the score. Although 50 per cent of those taking the tests had achieved government-set standards, that equalled only 35 per cent of those eligible to take them.
Roger Wicks, the solicitor acting for the school and the union, said: "As they stand, the tables are misleading. If this data creates an impression the school is underachieving ... some parents will want to send their children elsewhere, with the risk that the school will slip into a downward spiral."Reuse content