Ninety-eight per cent of headteachers say that this summer’s English spelling, punctuation and grammar test for 11-year-olds was more difficult than they expected, a survey by the NAHT heads’ union has found.

A poll of more than 2,600 NAHT members found wide-ranging concerns about the level of difficulty of this year’s Sats papers, the results of which will be published today. It is the first year in which reformed papers, designed to be tougher than the previous tests, have been used.

A third of respondents said the maths paper 2 for Year 6 pupils was “much more difficult than expected” and 24 per cent said the same about the maths paper 3 for key stage 2.

Some 88 per cent of respondents said they had “experienced issues relating to the difficulty of tests” and 81 per cent said pupils had not had enough time to complete some of the key stage 2 tests.

Almost all (98 per cent) said that this year’s key stage 2 Sats were not appropriate for children with special educational needs.

Children taking Sats in Year 6 this summer were the first to be tested on a new national curriculum that would normally be taught over a four-year period - but they had only had two years to study it.

Russell Hobby, general secretary of the NAHT, said today that this put current Year 6 pupils “at an automatic disadvantage”. Sixty-one per cent of respondents said that the lack of preparation time had a negative or strongly negative effect on their pupils’ ability to access the tests.

The NAHT survey also found major concerns about key stage 1 Sats. Eighty-two per cent said these tests were not appropriate for pupils with SEND.

Education secretary Nicky Morgan told the BBC yesterday that today’s Sats results should not be compared with previous years’ results.

“They simply cannot be compared directly,” she said.

Ms Morgan told the BBC that she expected “critics” of the new primary curriculum to “try and suggest that any lower results are evidence of a failure of the system”.

But she said: “Nothing could be further from the truth,” adding that this year’s tests were “based on a new, more rigorous national curriculum, based on the best evidence from across the world.”

The NAHT has called on ministers to avoid publishing this year’s Sats results, arguing that the move to a new system meant the data would be of little use.

But Mr Hobby said today: “The government does love a league table, regardless of how accurate it may be.”

James Bowen, director of NAHT Edge, the middle leaders’ union, said that Sats had “become a box-ticking exercise for children in order to satisfy bureaucrats and politicians.”

He said: “Increasingly, parents and teachers agree that high-stakes statutory tests like Sats can actually make it harder to find out what children are really learning and to improve their education.”

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