Officially, the regulations will allow staff who work less than 35 weeks a year, those on contracts of three months or less and casuals to join the scheme.
But principal pensions adviser for the Local Government Management Board Bob Gibbs said the DoE had not defined casuals in any of the regulations.
'I was given a verbal assurance that when the regulations came out there would be a definition of what a casual was. I have been told a casual is anybody who has not been defined but I don't accept that as a way of getting through the problem,' he said.
The move to open the pension scheme is part of the Local Government Pension Scheme Regulations 1995, which come into effect on 2 May.