A-level students are consistently being predicted the wrong grades, research has shown.
Just one in six (16 per cent) university applicants achieve the exam grade points that they were predicted to achieve by teachers or lecturers, based on their best three A-level results.
The study, published by the University and College Union (UCU), also shows that students are likely to receive more generous estimates on their performance.
Overall, 75 per cent of applicants were over-predicted - meaning their results were predicted to be higher than they actually achieved. Researchers also concluded that around 9 per cent of applicants had results under-predicted.
The study examines the top three predicted and actual grades of around 1.3 million students who sat A-levels between 2013 and 2015 and went on to university, applying through the UCAS admissions system.
It also suggests differences depending on social background - almost one in four (24 per cent) applicants from lower-income households were under-predicted in their results, UCU said, compared with a fifth (20 per cent) of those from wealthier homes.
Sally Hunt, UCU general secretary, 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.
"This report is a damning indictment on a broken system, not the hard-working teachers tasked with the impossible job of trying to make predictions.
"The results strongly support our call for a complete overhaul of the system, where students apply after they receive their results."
The union said it is calling for a move to a post-qualification admissions system, which would mean students applying to university with firm results, rather than predicted grades.
In February, UCAS chief Mary Curnock Cook suggested that teachers are intentionally bumping up students' predicted A-level grades to help them win places at top universities.
It comes as institutions are now "more flexible" with grade requirements amid intense competition to attract students, she said.
Speaking at a higher education conference, Ms Curnock Cook said: "I talk to a lot of schools and people who advise students and in the past I would have said 'surely you wouldn't be over-predicting your students on purpose?', and actually just this last summer really, I had teachers coming back to me and saying 'actually, yes we would'.
"I'll show you why, because actually, accepted applicants, the number who are being accepted with quite significant discounts on their offers and their predicted grades has grown quite a lot."
Over-prediction of grades has always occurred, Ms Curnock Cook said, but she indicated that it is becoming more common.