Lecturers are working an average of nine hours a week in unpaid overtime, equivalent to a £10,000 annual gift to employers, according to the Trades Union Congress.
TUC research indicates that lecturers in effect work for free for more than two months a year. If they worked all their unpaid hours in one stretch at the start of the year, they would begin being paid on March 9.
The TUC was also expected to reveal as The Times Higher went to press that college and university lecturers, along with school teachers, were at the head of the annual unpaid-work league table - ahead of doctors and lawyers and behind only those defined as "top managers" - in working through lunch hours, weekends and evenings.
The league table is published on February 24, the TUC's "Work Your Proper Hours" day, which marks the day when the average UK employee would start to get paid if they did all their unpaid overtime at the start of the year.
Figures disaggregated by the TUC for The Times Higher showed that 108,977 higher education staff reported working an average of nine hours unpaid overtime a week. This would be worth £10,216 a year.
Sally Hunt, general secretary of the Association of University Teachers, said: "It will come as little surprise to anyone in the higher education sector that lecturers top the most overtime worked league table.
"Lecturers' workloads have increased massively over the years despite pay decreasing in relative terms. I would love to think that university employers will use Friday as an opportunity to thank their staff for all that hard work and commitment, but their current style of dealing with the staff pay claim suggests otherwise."
Andy Pike, national official for universities at lectures' union Natfhe, said that it was telling that the industrial action over pay, planned by both Natfhe and the AUT, included work-to-rule in addition to one-day stoppages and an exam boycott.
Natfhe members will work to contract, which will see them taking all their annual leave and refusing to work more than 37 hours a week.
"It is a sad indictment that this alone will cause real problems for universities," Mr Pike said.
The TUC report says that while the average unpaid overtime fell by six minutes in the past year, to 7.4 hours a week, the sector as a whole (including school teachers, further education and higher education lecturers) told the Government's Labour Force Survey that staff did an average 11 hours and 36 minutes of unpaid work a week - worth £9,937 annually.
The TUC is urging staff to "take a stand" on Friday, asking workers to take a proper lunch break and to go home on time.
A spokesman for the University and Colleges Employers Association said:"We certainly support a healthy work-life balance for academic and support staff at all levels throughout the higher education sector and we would not want to encourage a long-hours culture.
"The Joint Negotiating Committee for Higher Education Staff, which consists of Ucea and all seven of the HE unions, produced a Wo rk-Life Balance Guidance for Higher Education Institutions . This is available on the Ucea website at www.ucea.ac.uk .
"According to Ucea's recruitment and retention survey conducted in 2005, workload was not a major factor in the retention of staff in the higher eduction sector," the Ucea spokesman said.