Thousands of students are set to receive the wrong GCSE mark this week under the new numerical grading system, experts have said, as they warn of its “alarming” consequences.

The proportion of pupils who will receive an unreliable grade for their English GCSE this Thursday is set to rise from 30 per cent to 45 per cent, statisticians predict, as they call for a system where students are awarded percentages rather than grades.

Robert Coe, a professor in Durham University’s School of Education, said that in some cases a child’s grade will be “not much more than chance”.