A group of steelworkers who lost their pensions more than 20 years ago have compared their plight with the Post Office Horizon scandal.
Allied Steel and Wire (ASW) plant staff in Cardiff lost their pensions in 2002 when the firm went bust.
Campaigners won the right to get up to 90% of their pensions back, but this does not increase year-on-year, leaving former workers worse off in real terms.
The UK government said it was important the scheme was financially sustainable.
It launched the scheme in 2007 after dozens of protests, with about 1,000 ASW workers in Cardiff, Belfast and Sheerness, Kent, losing pensions they had been paying into for decades.
One of those, John Benson, said: "I go to bed at night (and) and think... 'what have I done to deserve this?'
"We played by the rules. We put our trust in politicians in Westminster, and they betrayed that trust."
Because the new pensions do not increase year-on-year - a process called indexation - recent high inflation has meant the money is stretching less and less far.
Campaigners said there were "obvious" parallels with the Post Office scandal, where hundreds of sub-postmasters were convicted before their protests were listened to.
Plaid Cymru leader Rhun ap Iorwerth said: "When something is wrong, when something is unjust, government simply has to pay up."
'Strong moral case'
Conservative peer and former pensions minister Baroness Altman said the pensioners had a "strong moral case" but it would be hard for them to get what they want.
She said: "I don't know where the money is going to come from because every government minister is fighting for taxpayer funding for all sorts of things and no DWP (Department for Work and Pensions) minister nowadays could demand that the Treasury pay up. It would be difficult."
"I just wish I could see something realistic that I could expect to happen, to sort of wave a magic wand and get government, which has adamantly refused every single time to improve the financial assistance scheme benefits for everyone."
The DWP said its scheme ensured people got 90% of their pensions, subject to an overall cap.
It added: "It is important to ensure the pension protection system remains financially sustainable, providing a safety net for savers, and fairness to the taxpayer."