The study by Durham University researchers indicates pupils who ‘failed’ maths at A-level in 1988, getting a D or an E – then a fail grade – would have received a B in 2011.

Overall, grades went up by a tenth of a grade every year for the past 20 years – two full grades over the past two decades, the research reveals.

It also said those receiving a D or E grade in biology in 1988 would get a B today, while those getting a D in French would not receive an A or B.

The statistics, produced by the Centre for Evaluation and Monitoring at Durham University, were cited by education secretary Michael Gove at an event organised by the Office of Qualifications and Examinations Regulation (Ofqual) in London.

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Mr Gove said: “While we award As, Bs and Cs on the basis of criterion reached, is there a case for exploring where or not A*s should be allocated only to a fixed percentage of candidates?”

He also floated the idea of a ranking showing where students finished in the country overall. “Of course, you know that their work is capable of securing an A or an A*, but you also know how they’re ranked, depending on the subject,” Mr Gove said.

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His decision to “open up the debate” was welcomed by Robert Coe, director of the Centre for Evaluation and Monitoring, who presented his research to the event.

“I am delighted that the Secretary of State has publicly acknowledged that there may be a problem with the maintenance of standards, but I would like to see both Ofqual and the Secretary of State get to grips with solving the problem”, he said.

He added: “Different users of qualifications, such as universities and employers, have different needs, which can only be met by having a range of qualifications - not one-size-fits-all.

“A system that allows new qualifications to emerge in response to the demands of users, students or teachers is a healthy one.”

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Martin Stephen, former high master of St Paul’s School, writing in the Daily Telegraph, says “world-class universities … need to recruit from the top 15 per cent of the ability band”. Grade inflation means elite students “were allowed to slip off the examination radar and become indistinguishable from the next tier of ability down”, he argues.

jack.grove@tsleducation.com