Proposed changes to firefighters' pensions could cost the taxpayer £210 million and lead to a huge exodus from the industry's scheme, union leaders have warned.
The Fire Brigades Union (FBU) said one in four firefighters could leave the pension scheme, threatening a sharp drop in contributions.
The union said plans to raise contribution rates from 11% to 14%, and by up to 17% for firefighters promoted to officer grades, were a "massive gamble".
Changes to the Firefighters Pension Scheme were aimed at saving £73 million by 2014, but they could end up costing the Government £283 million in lost contributions, said the FBU.
A YouGov survey of 8,000 firefighters for the union found that one in four were considering opting out of the pension scheme.
FBU general secretary Matt Wrack said: "The savings the Government is hoping for are very likely to turn into much higher costs. A sharp hike in already high contributions and the other changes could be the tipping point which will see an exodus from the main pension scheme.
"The perverse outcome of the dash for savings to help pay for the budget deficit will be large losses. The viability of the scheme will be badly undermined, costing even more in the long-term.
"Firefighters already pay a huge contribution for their pension. The proposals to increase this alongside the other changes are causing huge concerns and anger.
"It makes no sense for the taxpayer to be paying more, for firefighters to be paying more and the scheme producing less. The Treasury needs to think again and not take a massive gamble with firefighter pensions and taxpayer money."
Planned changes to public sector pensions have already sparked a strike by up to 750,000 teachers, civil servants and other workers next Thursday and could lead to wider walkouts in the autumn.