A former steelworker says UK government inaction after 1,000 people lost their pensions more than 20 years ago is disgraceful.
Allied Steel and Wire (ASW) plant staff in Cardiff lost their pensions when the firm went bust in 2002.
Campaigners won the right to 90% of their pensions back, but this does not increase year-on-year.
The pensions minister said assessment of what was owed would be too expensive.
But John Benson said the Department for Work and Pension (DWP) was "trying to just cheat us out of what we paid for".
"It's a disgrace the way they've treated us. They've got to accept their responsibilities," he said.
"No matter how much this costs, this is our money. We paid for this indexation, we paid for this pension, we trusted government.
"They waste billions on vanity projects then say they can't afford this."
The campaign to get the pensions restored began after ASW went bust and workers lost their jobs, along with pensions they had been saving into for decades.
After dozens of protests, the UK government set up a scheme which ensured workers from companies such as ASW would get back up to 90% of their pensions.
But, over the years, those pensions have gone down in value because they are not linked to inflation.
The workers say they are entitled to have that amount made up to include inflation, and for it to be backdated.
Plaid Cymru took the workers' case to parliament and asked the DWP to assess the change in the value of pension payments, as a result of inflation, that were not index linked for Allied Steel and Wire pensioners since 2007.
'Betrayed by the government'
Hywel Williams MP, work and pensions spokesperson for the party, said the workers affected had "unjustly been denied their full pensions" for 22 years.
"Workers say they are only receiving around half of what is owed to them due to the UK government's scheme failing to protect contributions from inflation," he said, adding the refusal to make up the difference was "deeply disappointing" while blaming cost was "in bad taste".
"Despite playing by the rules, they were betrayed by the government.
"It's clear that the UK government is not learning any lessons from the Horizon Post Office scandal and continues to take the head-in-the-sand approach to injustice. I urge them to reconsider."
Paul Maynard, pensions minister, said the information required to carry out the assessment was "not readily available" and "to obtain it would incur disproportionate costs".
"The department has therefore made no such assessment and does not intend to do so, at this time," he said.
But in a letter to Mr Benson, seen by BBC Wales, he added: "I understand your frustration that the wheels move slowly... but I can assure you it is an issue which I am still very much working on."