Hundreds of primary school pupils across York, North and East Yorkshire will not sit national tests which teachers plan to boycott.

Participation in the action against tests for ten and 11-year-olds is set to involve as many as 75 per cent of schools in some areas.

If enough schools take part in the boycott, organised by the National Association of Head Teachers (NAHT) and the National Union of Teachers (NUT), it will be impossible to draw up league tables comparing schools, throwing the primary school assessment system into chaos.

The Key Stage Two national tests in English and maths, formerly known as SATs, are due to be taken by 600,000 children between Monday and Friday.

According to national figures, around 50 per cent of primary schools are taking part, but in parts of North Yorkshire the figure is higher.

Anne Swift, press officer for the North Yorkshire NUT said that schools had been working in clusters and that in urban and challenging areas the response was likely to be higher.

She said: “In Scarborough, the take up is around three-quarters, and schools serving towns have been like that. Schools in rural areas are more isolated and under different pressures.”

She said that in the schools where parents and governors have been consulted, they are backing the boycott. She added that the tests were “a huge unnecessary cost” which meant most of Year Six was spent preparing for exams, which could be replaced by a system of continual assessment.

Elaine Williams, spokesperson for North Yorkshire County Council, said the authority could not get involved, other than to offer advice, adding: “The authority is confident that schools will ensure that children in the county are not disadvantaged by the industrial action.”

A spokesperson for East Riding of Yorkshire Council said it had not heard from all of its 119 primary and junior schools, but 49 of them had confirmed they would not be doing the tests. A City of York Council spokesperson said the authority had not been informed whether or not any of its schools were taking part in the boycott.