Offers of a place on a university course should not be made on the basis of "highly inaccurate predicted grades" according to the University and College Union (UCU).
Currently, places on degree courses are allocated on what the student is expected to get at A-level, not what they actually achieve.
UCU claims that the UK is the only nation which uses predicted grades, with research suggesting as few as one in six A-level grade predictions is correct.
It says the system encourages the use of unconditional offers, which can lead to students working less hard for their A-levels and achieving lower than expected grades as a result.
The UCU report is based on research into university admissions in 29 countries, plus England, Wales and Northern Ireland.
It concludes that offers should be made once A-level results are revealed.
UCU general secretary Sally Hunt said: "We are alone in the world in using a system where students are offered university places based on highly inaccurate predicted grades.
"Unconditional offers have made a mockery of exams and led to inflated grade predictions, while putting students under enormous pressure to make a snap decision about their future.
"The simplest and fairest way to deal with these problems is for us to adopt a system of post-qualification admissions, where offers are based on actual achievement rather than estimated potential, as the rest of the world does.
"It's time for the Government to give the system the urgent overhaul it needs."
Organisations representing senior school staff have supported the report's findings.
Malcolm Trobe, from the Association of School and College Leaders, said: "ASCL echoes the concerns in this report about the use of predicted grades to award university places and calls on the Government to review the system urgently.
"Out of date and no longer fit for purpose, it is a historical quirk which is not mirrored in other countries and creates unnecessary problems.
"In particular, we are extremely concerned about the rising number of unconditional offers made to students before they have taken their A-levels.
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"This practice can demotivate students and lead to under-performance in these important qualifications which disadvantages them if prospective future employers take their A-level grades into account.
"Moving to a system of post-qualifications admissions would end the practice of unconditional offers.