The Treasury had not carried out a proper assessment of the impact of changes to public sector pension policy, according to a report.
The House of Commons public accounts committee found that a key feature of changes announced three years ago has yet to start.
A cost sharing and capping mechanism has been long deferred, without an implementation date set.
The cross-party group of MPs expressed concern that workers did not have a clear understanding of the value of their pensions because they were not given "clear and intelligible" information.
Committee chair Margaret Hodge said government projections of the future cost of public sector pensions suggested that changes agreed with unions in 2007/8 would bring "substantial" savings to the taxpayer.
"However, we are concerned that the Treasury has not tested the impact of the changes on some of the key assumptions underlying their cost projections," she said.
"We are also concerned that the Treasury has not set out clearly what level of spending it considers sustainable in the long term. Instead, officials appeared to define affordability on the basis of public perception.
"The Treasury expects the majority of savings to come from cost sharing and capping, a reform designed to ensure that employees bear a greater share of future costs.
"However, implementation has been deferred because of the Treasury's discount rate review, and remains on hold while the government consults on the recommendations put forward by the Hutton Commission."
The committee expressed concern that the Treasury had not properly assessed how changes to public sector pensions might lead to additional spending elsewhere, such as increasing demand for means-tested benefits.
Brian Strutton, national officer of the GMB union, said: "We agree with the PAC that the forecast for the existing reforms shows that these are sufficient to manage the taxpayer cost over the long term. The report demonstrates that reforms to public sector pensions agreed under the last government would achieve substantial savings for the taxpayer if allowed to proceed.
"The tragedy for taxpayers, pension savers and future generations is that the current government is seeking to impose new schemes on the public sector that workers can’t afford and won’t produce enough pension to pay one energy bill let alone buy food. In the meantime they have stopped previously agreed reforms which has already meant that £1bn of savings has been lost."
Teachers' trade unions hit out at the failure of the Treasury to explain exactly how they had decided what level of public pension spending was affordable.
Christine Blower, general secretary of the National Union of Teachers, said: "The PAC is right to criticise the government for proposing further changes without even having considered what is affordable. This is a policy based on nothing short of false assumption and spin."
Chris Keates, general secretary of the NASUWT, said the report confirmed their belief that the "attack on pensions is being driven by ideology rather than affordability".
"This damming report highlights the Treasury’s failure to have even the most basic figures in place on which to base its proposals for worsening public service pensions.
"This is a shocking indictment of the cavalier way it appears the Treasury behaves."
A Treasury spokesman said: "The government wants public service pensions to remain among the very best available.
"But as former work and pension secretary Lord Hutton’s independent report made clear, current pensions are unsustainable, with the average 60-year-old now living 10 years longer than they did in the 1970s."


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