Weeks before this year’s results are published, Mr Johnson will launch a pre-emptive strike against critics of the ever-rising pass rate by insisting that pupils are simply getting better at exams.

Since Labour came to power the proportion of young people getting five good GCSE passes has risen by 6.8 percentage points from 54.4 per cent in 1997 to 61.2 per cent in 2005.

Last summer almost a quarter of all students were awarded one A grade or more at A level.

Universities complain that they can no longer discriminate between the bright and the brightest. Pupils, teachers and examiners insist that the exams have not got any easier despite the improvement in results.

Now the Government claims that research carried out to investigate the writing skills of 16-year-olds proves that children’s achievements have improved over the past decade.

Mr Johnson will today tell the UK Youth Parliament meeting at Leicester University that young people must be praised for excelling in exams. “We should be celebrating the fact that pass rates are going up and attainment is rising,” he will say. “Despite the received wisdom of those that seek to detract from the achievements of our young people, research shows young people’s performance is improving.”

The minister will cite a report called Variations in Aspects of Writing Between 1980 and 2004 as evidence that the crucial skills of punctuation, grammar and vocabulary have risen in the past decade.

The study, by Cambridge Assessment, examined sentences written by 1,779 teenagers in the creative writing sections of English exams in 1980, 1993, 1994 and 2004.

The academics found that today’s GCSE pupils have a better mastery of written English than a decade ago and that pupils use a wider range of vocabulary and have a better grasp of grammar.

The report’s authors said: “This evidence of improvement in skills which are fundamental to academic work in all subjects — not just English — should prove very welcome to all concerned in education.”

However, Alan Smithers, director of education and employment research at the University of Buckingham, said that the minister was “fooling himself” over the grades being achieved in the exams.

Professor Smithers said that independent studies had shown “no, or slight, improvements” in student achievement, and that this research did not support Mr Johnson’s claims.

The report, which was published last year, looked specifically at vocabulary, spelling, punctuation, sentence structure and the use of non-standard English. However, the authors said that investigating “vital qualitative features of writing, such as imagination, content and style” was impossible on the evidence.

Sylvia Green, director of research at Cambridge Assessment, said that the study had found a dip in performance in the 1980s, but that during the 1990s punctuation, grammar and vocabulary had returned to previous standards.

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