Sixth-formers are hoping their A-level results would be good enough to secure their place at university.
But despite the pass rate likely to rise again this summer, many students will find their grades are not good enough to secure a university place.
Tens of thousands of youngsters, many with top grades, will be disappointed, universities minister David Willetts has admitted.
And many of the UK's top universities have warned they are already full, and the clearing process, which matches students with available courses, is expected to be short.
Thursday's results are expected to show that more than one in four entries will be awarded an A grade.
Sweeping changes to A-levels come into effect this year, with the new A* grade due to be awarded for the first time. It has been estimated that one in 14 entries could be awarded the grade.
More than 250,000 teenagers in England, Wales and Northern Ireland are expecting results, but many could find that good grades are not a passport to a degree course.
It has been predicted that between 170,000 and 200,000 students, including sixth-formers and older learners, could miss out this autumn as universities face multi-million pound cuts and pressure on places.
By the end of June more than 660,000 people had applied to start full-time undergraduate university course.
Carl Gilleard, chief executive of the Association of Graduate Recruiters, said the clearing process would be "more stressful than ever". Students should consider their options before making any decision, he said.
