It will cost more than £750 million to recruit the minimum of 22,000 new lecturers needed to hit the government's higher education expansion target by 2010, according to the Association of University Teachers.
The AUT said this week that the investment was needed over eight years. It would be in addition to the £840 million a year the union says is needed to increase salaries for existing staff and to close the gender pay gap.
The union said the figure of 22,000 was a "conservative" estimate based on keeping the current ratio between staff and students at 1:18. Additional problems such as early retirement and falling recruitment in some academic disciplines would require more staff and possibly more money.
Sally Hunt, assistant general secretary of the AUT, said: "If ministers are to successfully meet the target, then there is only one option open to them in the forthcoming spending review: to invest more money in university staff."
The AUT's figures are based on projections from the Department for Education and Skills. The DFES estimates that 400,000 more students must get into higher education to meet Tony Blair's 50 per cent target by the end of the decade. It is thought that the total cost of recruiting and teaching these students could run to billions of pounds over the period of the planned expansion, 1999-2000 to 2010-11.
The AUT said that the additional student numbers required 22,222 new staff. At current prices, using the average salary for academic staff combined with employer's pension costs, this would mean an extra £750 million, or about £100 million more each year until 2010.
The union acknowledged that its calculations were crude, as they did not factor in academic-related staff recruitment or the numbers of new students in further education or part-time study.
But the AUT said its estimates were conservative because of the many lecturers approaching retirement age in the next few years. The most conservative projection, based on the unlikely event that most of those staff work until they are 65, means that 17,285 new lecturers will be needed to replace those retiring.
A report by the AUT shows that the average retirement age for staff in old universities is 60. In 1994, 22 per cent of all academic staff were aged 50 and above. This had increased to 28 per cent by 1999-2000.
Ms Hunt said: "The time bomb hidden in the age profile of university staff has a short fuse. By 2010, the combined effect of early retirement and increasing students numbers will be a major embarrassment to institutions unless there is long-term investment."
The AUT accepted that many of the additional students might study only part time and that many of the new groups of students would require more support than the traditional middle-class undergraduates.