The Labour leader Brendan Howlin says reducing the pensions of some public servants may be unconstitutional.
Mr Howlin's claim came in a speech at the party's annual James Connolly memorial in Dublin today.
Last week, the Public Service Pay Commission said most public servants should contribute more towards the cost of their pensions.
In his speech, Mr Howlin claimed that is unjustified, and that new entrants already had reduced entitlements.
He also said "attacking" the pensions of public servants who are nearing retirement age would be "immoral".