Primary test results in English have stalled for the third year running because pupils are bored by a narrow curriculum, headteachers say.
Results from a sample of schools returned to government advisers show that 11-year-olds have failed to progress towards ministers' targets either in English or maths.
In previous years, results from the sample have closely mirrored the nationwide picture which will be known next month.
Last year 75 per cent of pupils reached level 4 in English and 73 per cent in maths. After early gains English results have stalled at 75 per cent since 2000.
This year ministers downgraded the national target of 85 per cent of 11-year-olds achieving level 4 by 2004 to an aspiration to be met in 2006.
At a press conference this week, Michael Barber, head of Downing Street's delivery unit, said that in primary education a "plateau" had been reached in English and maths, but recent international data showed Britain was now sustaining a "world-class performance".
Professor Barber once said that the Government aimed to ensure that all pupils could read and write.
The tests were altered this year in an attempt to counter "teaching to the test". Children could no longer choose which question to answer in the writing paper and in maths and science more emphasis was placed on problem-solving.
The changes, although welcomed, led to fears that results might dip. David Hart, general secretary of the National Association of Head Teachers, said the fact that results had been maintained should be seen as a tribute to teachers who had warned they were inadequately prepared for the changes.
The Qualifications and Curriculum Authority has said that the tests were more difficult this year than in previous years. It lowered its boundary for a level 4 from last year's 49 marks out of 100 to 44 marks.
Mr Hart said: "In the short term, the results are due to changes to these tests. One other major reason is a government policy that assumes the problems with the primary sector have been solved.
"The primary sector still suffers from a massive discrimination in funding.
Many schools do not have a broad and balanced curriculum. I think this handicaps schools when it comes to English, maths and science because achievement in non-core subjects undoubtedly has a positive spin-off on other subjects."
Peter Frost, chief executive of the National Primary Trust, said: "If we continue on the same course it may be possible to get marginally more children to level 4, but I think we are very close to saturation point."
The Government announced its revamp of the primary curriculum in the week after the key stage 2 tests. Its Excellence and Enjoyment strategy has urged schools to be more creative and innovative.
* speaking and listening
* to strengthen teaching of early phonics and continue this into Years 2,3 and 4
* to give children a wider choice of texts.
Maths: to ensure children
* can apply mathematical skills to different situations
* understand how numbers work
* are able to solve increasingly complex problems
* can use maths to reason and explain.