Most students' predicted A-levels 'wrong'
Only 16% of university applicants achieve the grades their teachers predict, research suggests.
Analysis of the results of 1.3 million young people over a three-year period found 75% had been given overly optimistic predictions by schools.
But nearly one in 10 (9%) did better than predicted, the study, published by the University and College Union, says.
University admissions service Ucas said the 16% related to those with no net deviation from all their predictions.
The UCU is calling for an overhaul of the university admissions system, which currently sees students apply on the strength of their predicted grades.
It said it was time the UK allowed students to apply with firm results not predictions that are "poor guestimates".
It said a post-qualifications admission (PQA) system would also abolish the need for unconditional offers for university places.
Researchers at UCL's Institute of Education analysed the top three A-level results from 1.3 million candidates who sat A-levels in 2013, 2014 and 2015 went on to higher education through the Ucas service.
The report also found the grades of able students from disadvantaged backgrounds were most likely to be under-predicted.
Almost one in four (24%) applicants from lower-income households was under-predicted in their results, the UCU said, compared with a fifth (20%) of those from wealthier homes.
Report author, Dr Gill Wyness, from UCL's Institute of Education, said students having their future grades under-rated by teachers should be of particular concern.
She said: "I find worrying evidence that, among high-achieving (ie 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."
Dr Wyness said applicants who were under-predicted were more likely to apply to, and attend, a university for which they were over-qualified, which could, in turn, have an impact on their future careers.
UCU general secretary Sally Hunt 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.
"It is quite absurd that the UK is the only country that persists with using such a broken system."
But UCAS chief Mary Curnock Cook rejected the UCU's calls.
"Whilst a post-results application system is logical, it would work against those from less advantaged backgrounds," she said.
"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.
"Finally, it is not the case that only 16% of predicted grades are right - the correct interpretation is that only about 16% of students have no net deviation at all from their predicted grades across three A-level results."
A Department for Education spokeswoman said it was up to universities, as independent bodies, to decide on their admissions processes.
"However we would expect them to take account of the full range of information available, not just predicted grades, so that they select the students with the talent and potential to succeed on their particular course," the spokeswoman added.