The decision to raise the pension age will hit tens of thousands of people who have taken early retirement, some of whom will be faced with a shortfall of up to €20,000, according to trade unions.
Four unions affiliated to the FNV federation are planning to take the government to court to win compensation for the people who will lose out.
The state pension age rose from 65 to 65 plus one month in January and will rise in stages to 67 by 2023.
But, the unions say, people who have already retired early will be left with a funding gap. This is because their early leaving package stops when they hit 65, not 65 and a few months.
The social affairs ministry is working on measures to help retirees bridge the gap but the unions say the current offer is not enough.