Cambridge University has written to the Education secretary, Michael Gove, criticising his plan to reform A-levels in England.
Sixth-formers usually take four or five AS-levels in the first year, before specialising in A2s in the second year.
Michael Gove wants to return to one set of exams after two years to revive what he calls the "art of deep thought".
But in the letter, Cambridge admissions manager Geoff Parks says AS-levels are an "invaluable indicator of progress".
He says that the exams have helped universities choose the best students and have widened participation by giving bright students from less privileged backgrounds the confidence to apply for top universities.
The results of the first stage of A-levels - AS levels - are available before universities make offers to students.
Mr Parks told the Today programme on BBC Radio Four: "We have discovered from the process of admitting students post the last set of A-level reforms that information given by the AS-levels is immensely valuable in assisting both admissions tutors in selecting the most able candidates but also in providing encouragement to able students from non-traditional backgrounds to realise that they have the potential to apply to the top universities in the country."
Modular A-levels have been criticised as easier than the traditional variety.
Critics have said they break up study into too many units, limiting the coherence of a course and the depth of understanding reached by students.
Announcing plans to change the system earlier this month, Mr Gove said universities had complained that A-levels were not preparing students sufficiently well and that he wanted them to be more academically rigorous.
In his letter, seen by the Times Educational Supplement, Mr Parks warns that "admissions tutors in Cambridge have read with interest and a degree of anxiety the reports of your recent comments about A-level reform".
It says: "We are worried... that if AS-level disappears, we will lose many of the gains in terms of fair admissions and widening participation that we have made in the last decade."
The letter says that the grades obtained provide a better guide to future performance of students as Cambridge undergraduates than other methods of selection, such as GCSE results - which are "nowhere near as reliable" - or aptitude tests.
Mr Parks added that the proportion of Cambridge places awarded to students from the state sector and other "under-represented groups" had risen in recent years.
"We are convinced that a large part of this success derives from the confidence engendered in students from non-traditional backgrounds when they achieve high examination grades at the end of year 12," he wrote.
Mr Parks told the Today programme the university shared some of Mr Gove's concerns about A-levels.
"We agree with the secretary of state in some aspects of the reform. We would agree that at the moment A-levels are too modular and there is too much examination. We are not sure all the opportunities for re-takes are advantageous.
"But we think there is a middle way between where we are now and the proposals he has set out."
The university has not yet received a reply to its letter.
A Department for Education spokesman said there was a need to restore confidence in public exams.
"We welcome Cambridge University's views on the A-level and look forward to working with them and other universities to ensure these vital exams are robust and rigorous and properly prepare students for higher education.
"It is right that in doing this we should consider the effect that splitting courses into modules has had both on results and on the exams' continuing fitness for purpose.
"Nothing has been decided on changes yet and we shall set out detailed next steps on reform later this year."