The extension of pension rights to 120,000 part-time lecturers has brought fears of a "horrendous" cash squeeze on colleges and new universities.
Unions are urging hourly-paid lecturers to benefit from their new eligibility to join the Teachers' Superannuation Scheme.
The Department for Education announced that restrictions would be lifted from May 1 following a European Court ruling that excluding part-timers from pension schemes amounted to sexual discrimination, since most are women.
The Scottish Office and Department of Education in Northern Ireland have also changed their rules.
Employers say the extra cost burden will hurt but cannot calculate the full impact until the Government's imminent announcement on how far claims can be backdated.
The earliest possible date is 1976 but a two-year retrospective ruling is more likely.
"We are deeply concerned about it because the cost could be horrendous," said a spokesman for the Universities' and Colleges' Employers' Forum. Institutions contribute 8.5 per cent of each pension-holder's salary to the Teachers' Superannuation Scheme and employees 6 per cent. He said: "If it goes back to 1976, it would just be an impossible position. It would be a bill we just could not afford.
"We will be putting a case to the Government about funding of any retrospective awards because the sector simply could not afford to pay."
Colleges' Employers' Forum chief executive Roger Ward said: "While we welcome appropriate improvements in part-timers' conditions of employment, we have to be mindful of the ever-increasing cost burden on colleges who are already faced with severe cash problems.
But Sue Johnson, head of pensions at the Association of Teachers and Lecturers, said she did not want anyone to miss out on the chance to join the TSS.
She added: "I am concerned that people who are eligible to join the scheme will not realise they can.
"For a part-timer, the sooner they join the better. They have already lost out on years of benefits."