The “unreliability” of GCSE and A-level exam grades has been branded “dangerous” and “scary” by a leader of the independent schools sector.

Mike Buchanan, the executive director of the Headmasters’ and Headmistresses’ Conference (HMC) highlighted research from exams regulator Ofqual about the consistency of exam marking.

A report published last November found that the probability of a candidate receiving the “definitive” grade - the grade given by senior examiners - varies from 96 per cent in maths to 52 per cent of English.

Quick read: Marking consistency varies across subjects, finds Ofqual

Need to know: Are nearly half of GCSE and A-level grades wrong?

Profile: Meet the exams regulator who messed up his A levels

Speaking at today’s International Festival of Learning at West Suffolk College, Mr Buchanan said the problem was that while marking was “fuzzy”, with a range of marks for a particular answer often deemed acceptable, grade boundaries are “hard”.

He said that this meant many “unlucky” pupils received grades lower than they deserved, and added: “It’s scary stuff. Most of the time this works very well, but in the system there is a systematic problem.”

Mr Buchanan cited an Ofqual finding that across all qualifications, the probability of receiving either the definitive grade, or the grade immediately higher or lower, was 95 per cent.

He said: “That should be our approach as employers, as people trying to select students for university or apprenticeships or sixth forms: plus or minus one [grade] we can be confident of, but if you’re saying ‘you have got to have this’ and you think that’s definitive, that’s a really dangerous thing.

“And it’s no good for the generality of our society. We are missing talent because we are relying on things to be definitive when they are not.”

Mr Buchanan called for an information campaign to inform people about what he called the “unreliability of grades”.

He said: “When I go out and talk to people about this, even in the teaching profession, most people are not aware of the unreliability of grades, and when you talk to employers or employers’ organisations and say ‘are you aware’, their jaws just drop.

“Universities, are you aware? Absolutely not.

“Part of what we are doing is trying to raise awareness of the issue, and at the very least to get people to use these grades not as the only definitive measure that they have of that person, because they are not definitive.”