Universities are refusing to reveal secret details of wage rises for vice-chancellors, according to a report into the "murky world" of senior academic pay.
The University and College union (UCU) said just 27 out of 139 institutions responded to a request to see minutes of meetings of remuneration committee, which set pay rates, while half of those that did reply redacted some information.
The union, which has staged a series of strikes since last October after claiming that university staff had suffered a 13% cut in real-terms pay since 2009, said it was time to "lift the lid" on pay rises for senior staff.
Two thirds of universities "snubbed" the request for minutes, often citing confidentiality, while a further 15% did not respond at all.
UCU general secretary, Sally Hunt, said: "Millions of pounds of public money are spent on vice-chancellors' salaries, yet their pay rise is decided behind closed doors with no accountability.
"The time has come for the lid to be lifted on the hitherto murky world of remuneration committees and senior pay in our universities. Students are paying £9,000 a year and they, and the taxpayer, have a right to know why so much of their money is going on paying the vice-chancellor.
"All but five university vice-chancellors earned more than the Prime Minister last year, while staff have been on strike six times this year in protest at a measly 1% pay offer. We believe there is a strong and legitimate public interest to justify these growing six-figure salaries."
Unions representing university staff are in dispute over pay, and the UCU is planning a marking boycott of exams and student course work from April 28 if talks next week do not break the deadlock.
The UCU said average vice-chancellors' pay was over £250,000.