The guidance, from the OCR exam board, has prompted one teacher to suggest that convenience for the awarding body is being put ahead of fairness to students.

John Dunford, general secretary of the Association of School and College Leaders, said: “If the exam board is trying to make it difficult for examiners to change grades, then that’s not fair on the students who had their paper badly marked the first time.”

The guidance came in a letter sent last month from OCR to examiners who conduct A-level re-marks, and seen by The TES.

It says: “When scripts are re-marked, additional marks should be awarded (or marks deducted) only when a demonstrable error is found in the application of the marking scheme.

“If a degree of subjectivity is involved, very strong grounds are needed to change the original mark. If you are broadly in agreement with the mark, please do not change it.”

It says that examiners who change a mark on the paper after a challenge from a school or college must complete a “justification report”. “Examiners must provide details to support the statement why the mark scheme has been inappropriately applied,” it says.

Juliet O’Callaghan, head of psychology at Wooton Upper School in Bedford, was sent the guidance by mistake by the board during a dispute with it over marking.

She said: “This is putting pressure on the marker to shift the grades only a small bit, if at all.

An OCR spokesman said reviews of marking were meant as a check on whether the original marking was correct.

He added: “In any subject... there is a relatively small degree of tolerance within which examiners may quite reasonably differ as to the mark given. Therefore unless the examiner carrying out the review believes that there has been an error in the orginal marking, that marking should stand.”

A QCA spokesman said that OCR had not breached its code of practice.