The Qualifications and Curriculum Authority also admitted that the marking of English tests for 14-year-olds, taken by 600,000 pupils, is poorer than for maths and science exams.
The English test fiasco happened even though ministers poured more money into marking last year. They had expected the number of schools challenging results to fall.
Charles Clarke, the former education secretary later told the QCA that the handling of the tests, which led to hundreds of schools complaining of late and inaccurate results, had "significantly damaged" their credibility, the documents show.
The revelations emerge in correspondence between the QCA and the Government obtained under the Freedom of Information Act.
Ken Boston, QCA chief executive, wrote to David Miliband, then schools minister, on June 3 last year about marking. He wrote: "I am aware that you have written to me on more than one occasion expressing your concerns about the quality of English national curriculum test marking at key stages 2 and 3."
Ministers had wanted to set the QCA a target of reducing the number of pupils' results in KS3 English changed as a result of school appeals from 3.8 per cent in 2003 to 2 per cent last year. However the QCA only agreed to a target of 3 per cent. In the letter, Dr Boston explained why he wanted 3 per cent. He wrote: "As you know, there are considerable differences in the quality of marking between the science, mathematics and English tests."
There were far fewer appeals for maths and science than for English. For KS3 English, teachers were unwilling to trust external markers to mark accurately, he said.
Final statistics for the 2004 KS3 English tests show that the number of "reviews" of pupil results requested by schools rose by 70 per cent, from 7,272 in 2003 to 12,433. The QCA said that 3.7 per cent of all pupils'
marks were changed as a result.