Some 75% of the children who took spelling, reading and writing tests reached the required standard.

Of these, just 12% spelt "thoroughly" correctly, while "rehearsed" was beyond 67.5%, the Qualifications and Curriculum Authority found.

The QCA analysed a sample of 215 successful candidates' test papers.

Back to basics?

It found more than 80% had spelled words such as "together", "important" and "involved" correctly.

But "qualified" proved too hard for four out of 10 pupils.

Only 43% spelled "effortless" correctly, while just 40% had "knowledge" right, with the silent "k" and the "dg" catching out many pupils.

The analysis showed 61% spelled "participate" wrongly, with the "c" causing problems.

Nick Seaton, chairman of the Campaign for Real Education, said: "I think that parents and employers will be absolutely horrified that more than half of 11 year olds haven't learned to spell a fairly regular word like 'effortless'.

"I would have thought that these are the very things that primary schools should be doing, although they say they are concentrating on the basics."

The government had set a target that by 2002 80% of 11 year olds should have reached "Level 4", regarded as a pass mark.

However, the proportion that did so in 2003 remained at 75%, not having changed since 2000.