The University of Manchester is asking current and former staff to give back thousands of pounds it says they have been wrongly paid.
An internal audit discovered that 1,073 people were overpaid a total of £590,556. The university blamed the error on turbulence after its merger with the University of Manchester Institute of Science and Technology.
A spokesman said: "As part of the merger process, a large and complex exercise began to negotiate a new pay and grading structure to meet the requirements of the (2004) National Framework Agreement.
"Interim arrangements were put in place to enable staff to be paid on the proposed new pay spine until a final agreement could be reached. Based on these interim arrangements, back pay dating to October 2004 was calculated and awarded to staff.
"It was subsequently discovered that for a number of staff there were flaws in the calculations and they had therefore been paid incorrectly," the spokesman said. "In instances where overpayment errors have come to light, the university has sought to inform staff as quickly as possible and to agree repayment plans that reflect an individual's personal circumstances. This has been carried out with the full agreement of the campus trade unions." He said that by the end of June, 570 people still owed £294,486.
Times Higher Education understands some individuals have been asked to repay more than £1,000.
At least one former employee, who has been sent a demand for more than £600, disagrees with the calculations. The demand said she should have received back pay at level 6, when she maintains that her post was covered by National Health Service funds at a level 7 salary. "Just how desperate is this institution for cash?" she asked.
A University and College Union spokesperson at Manchester said: "I can confirm that there have been some errors in pay calculations. This was at the time of the interim stage of our pay and regrading exercise. We have obviously been concerned about this and have done our best to advise and represent our affected members."