Standards, particularly in literacy, have fallen for children who were taught while the primary national curriculum was being introduced, according to a survey of secondary schools.
The Secondary Heads Association says that in more than two-thirds of comprehensives and one third of independent schools the attainment of pupils, measured on entry, has fallen in recent years.
The union blames the implementation of the 10-subject curriculum for squeezing the time available to teach reading, comprehension and spelling, but notes that standards in maths have not been hit in the same way as literacy.
John Dunford, SHA's president, says the new slimmed-down curriculum, devised by Sir Ron Dearing and introduced this year, had already helped, but the problems could persist until 1997.
Mr Dunford says he has noticed falling standards among new pupils at his school, Durham Johnston, in Durham, and discovered other heads were saying the same.
Last summer SHA surveyed 455 schools. Its conclusions are based upon the results of tests, usually ones devised by the National Foundation for Educational Research, given to pupils on entry. More than half the respondents had used these in-house tests for five years or more.
Of the comprehensives able to make a comparison, two-thirds (106) say intake scores have dropped, with one-third of independent schools (14) agreeing. Of those surveyed but unable to make comparisons, almost one half blames lack of data and nearly a third say their catchment area has changed within the period considered.
Mr Dunford says responsibility for falling standards lies with Kenneth Baker, the former education secretary responsible for the original curriculum, and Duncan Graham, former chairman of the National Curriculum Council and his successor Chris Woodhead (now the chief inspector). Their "relentless enthusiasm" in implementing the curriculum had caused the problems.
He says: "Secondary schools will do their best to ensure that the cohorts of pupils who have entered in recent years will not suffer, but we shall need the resources to help us to respond to this challenge."
John Sutton, SHA's general secretary, says the findings are a warning against shifting cash into the primary sector when resources for remedial education are needed in secondaries. He says the results will also be useful in the assessment of value-added statistics.
The falling standards are also a source of worry for heads as the Government wants 85 per cent of pupils to gain five or more GCSEs, grades A to C, by the year 2000.
However, the survey has been met with scepticism by the School Curriculum and Assessment Authority. It questions the extent to which the figures can be extrapolated. The validity of the tests themselves have also been called into question, as they may not cater for the fact that what children are learning has changed.
Mr Dunford says the union had called for an organisation such as the NFER to do a more thorough survey on the subject.
Ralph Tabberer, assistant director of the NFER, says national monitoring, beyond the curriculum tests is needed. He says it is difficult to tell from the data whether falling standards are a new trend.
In the early 1990s the NFER, following similar reports of declining reading ability, did research which found that while the more able children were keeping up with their reading the less able were lagging. It concluded that more research was needed into the effect of home, pre-school and school in relation to children's literacy.
The results of the survey were rejected by the National Association of Head Teachers, which represents most primary heads. David Hart, general secretary, says it is flawed and would be interpreted as an attack on primary schools.
He says: "SHA may not be holding primary schools responsible for the alleged decline, but others will. It is highly regrettable that SHA's survey will be interpreted as an attack on primary schools at a time when both sectors should be working together to meet the common objective, an improvement in overall standards."