Margaret Thatcher attempted to put off the introduction of GCSEs because she feared the exams would lead to lower standards and a “can’t fail” mentality among pupils, newly released files show.

In comments which are likely to be seen by Conservatives as a further vindication of their sweeping reforms of the exams, the then prime minister said the system would allow results to be distorted by “biased” teachers helping teenagers with coursework.

Previously unseen papers show Mrs Thatcher warned six months before GCSEs replaced O-levels in 1986 that she did not “like the sound of the new exam”, and asked for its introduction to be delayed.

However she eventually concluded that to intervene would amount to a public “contradiction” of Sir Keith Joseph, the education secretary and a close friend, and appear as if she was taking the side of teaching unions, which wanted more time to prepare for the new system. She therefore had “no option but to go ahead”, she told aides.

Her previously unknown concerns are revealed in official papers from 1986 released by the National Archive in Kew, west London.

Her fears appear to chime with the views of Conservatives about the GCSE system today - almost 30 years later.

Michael Gove, who was education secretary until the summer, is said to believe that the introduction of the exam was a “historic mistake” that has led to a dramatic fall in standards. Before stepping aside to become Chief Whip he set in train an overhaul of the curriculum which he said would address “the pernicious damage caused by grade inflation and dumbing down”.

GCSEs were eventually introduced in September 1986 with Kenneth Baker, now Lord Baker of Dorking, as education secretary, after Sir Keith stepped down in May - weeks after his clash with Mrs Thatcher over the exams.

Sir Keith had insisted that the new scheme was intended to better stretch teenagers, creating a tougher, “clearer and fairer” system. Under the old system, as part of which more academic teenagers took O-levels and others took CSEs, individual grades were awarded largely based on the relative performance of competing candidates.

GCSEs were intended to ensure a focus on “how much or how little pupils understand, know and can do”.

Mrs Thatcher, herself a former education secretary, raised her concerns about GCSEs with Mr Joseph in the spring of 1986. However on March 6 Mr Joseph wrote to her insisting she was “misleading herself” about the exams.

He insisted that the new system would “inject more rigour” and “use-able learning” and would be “a key instrument for improving standards”.

Mrs Thatcher marked his three-page briefing note with a series of hand-written annotations, complaining about his use of “an awful lot of high language” and questioning a number of his claims.

Sir Keith said that under the O-level and CSE system pupils were “simply ranked in merit order, with little regard to how much or how little pupils understand, know and can do”.

But in a hand-written annotation to his letter Mrs Thatcher said: “This is not a correct judgement of the present examinations system. We were taught to think and apply 50 years ago.”

Mrs Thatcher was advised by the No 10 policy unit to postpone the new system until it was clear that it was “workable”. In one briefing note she was warned that “GCSE is an exam nobody will fail” and “does little for the lowest 30 per cent of students”. Implementing the new system in September was a “hopelessly unrealistic” prospect, an official said.

In a summary of Mrs Thatcher’s concerns, dated March 18 1986, Mark Addison, then her private secretary for home affairs, said the prime minister believed the new approach would lead to “lower standards; a shift away from the traditional approach to learning in favour of a ‘can’t fail’ mentality; assessment by the pupils’ own teachers with the consequent risk of introducing more bias.”

Mrs Thatcher had not, Mr Addison added, been impressed “by the jargon-soaked justifications” of the exam produced by Mr Joseph’s department.

She asked for implementation of the exam to be postponed for at least a year, in line with the demands of many teachers, to help ensure the syllabuses were “sufficiently rigorous” and the coursework “properly assessed”.

However in early April she acquiesced with Mr Joseph’s insistence that the Government should hold its course, agreeing with Tim Flesher, another of her private secretaries, that to back down would “look like taking the side of the unions”. “I agree - no option but to go ahead,” she said.

Asked about the disclosures, Lord Baker told the Telegraph: "She was concerned ... because she always felt that whenever you change anything in education it might be for the worse."

However, he added: "In defence of Keith I don't think she fully appreciated that the great thing about GCSEs is it did away with CSE, which was virtually valueless."

The standard of GCSEs was initially "high", Lord Baker added, saying there had been a "degeneration" in grades over time.