A three-year analysis of 1.3 million UK students’ results found that three quarters of young people had been given predictions that overestimated their results.
Research from the University and College Union (UCU) suggests that only 16 per cent of A-level students achieve the grades predicted by their teachers.
The report also found that 9 per cent of students exceeded the predictions that were set by their teachers, while students from disadvantaged backgrounds were far more likely to have their grades under-predicted.
24 per cent of students who came from less wealthy backgrounds had predicted grades lower than their final results, compared to 20 per cent of those from more well-off families.
The UCU is requesting a change to the current admission policy, and has said the UK should allow prospective university students to apply using their final results, rather than the current system which can be inaccurate.
Dr Gill Wyness, from the Institute of Education at University College London and lead author of the report, believes under-predicted grades should be a major concern.
She said: “I find worrying evidence that, among high-achieving (i.e. AAB or higher) applicants, disadvantaged students are more likely to be under-predicted than their more advantaged counterparts.
“Indeed, almost 3,000 disadvantaged, high-achieving students (or 1,000 per year) have their grades under-predicted.”
Sally Hunt, general secretary of the UCU, said: “This report exposes the vast majority of predicted grades as guestimates, which are not fit to be the basis on which young people and universities take key decisions about their futures.”
However, the current system was backed by Mary Curnock Cook, the UCAS chief, who said: “Whilst a post-results application system is logical, it would work against those from less advantaged backgrounds.
“It wouldn’t leave enough time for universities to properly assess and meet the needs of the full range of students, nor for students (particularly from disadvantaged backgrounds) to conduct all their research into accommodation and finance before making informed choices.”