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A shake-up of the exams system makes it all the more important for teenagers to think carefully about their A-level choices, leading universities say.

Sixth-formers now have fewer chances to try a subject at AS-level before committing to taking the full A-level.

This means their subject choices carry more weight, according to the Russell Group, which represents 24 research-intense UK institutions.

The warning came as the group published its updated Informed Choices guide.

The guidance gives students and schools advice on the subjects and subject combinations that are most useful for courses offered by Russell Group universities.

"Up to 15% fewer students are taking AS-levels after changes mean they no longer count toward A-level grades," the Russell Group said.

"Students therefore may have less opportunity to try a subject before taking it to A-level and their choices now bear more weight."

Disadvantaged students

Director general of the group, Dr Wendy Piatt, said: "We are wholeheartedly committed to ensuring our doors are wide open to talented and able students from all backgrounds, but our universities can't offer places to those who do not apply or do not have the right grades in the right subjects.

"Too often, students disadvantage themselves by choosing a combination of subjects at A-level that will not equip them with the appropriate skills and knowledge for their preferred university course.

"Subject choice at GCSE and A-level or equivalent affects everyone's options for degree courses, so we make sure that the most useful and relevant information is available to students everywhere, through Informed Choices."

In April, the consumer group Which? said too many university applicants realised too late that they had chosen the wrong subjects at age 16.

The consumer group surveyed more than 1,000 UK 18- and 19-year-old university applicants and found almost a third (28%) said they wished they had chosen different subjects and 41% wished they had considered which subjects would be of most use.

Under a major overhaul, AS-levels have been decoupled from A-levels to form a standalone qualification.

New tougher two-year A-level courses, with exams at the end, are being introduced over a three-year period.