If the claims are successful they are likely to cost the authority more than £1 million, and could lead to further applications for surcharges against current and past Westminster members.
The new claims result from a recent decision by ombudsman Julian Farrand that a former employee of Westminster was receiving less pension than Westminster was obliged to pay him.
Westminster is contesting Mr Farrand's decision, and was informed by the district auditor that it had no powers to pay the enhanced redundancy payments, including compensation pensions, that had been offered to some staff from 1981.
Until this initial case is decided no further judgments will be reached by Mr Farrand, but he is currently reviewing at least four more Westminster pensioners' cases, with others lodged.
Both Unison and the Westminster Pensioners Action Committee, which represents around 200 of nearly 600 pensioners affected, are to write to members in the next few days advising them to lodge claims with the pensions ombudsman by the middle of January, when the three-year limit for review expires.