The deficit in the Local Government Pension Scheme in England may have more than doubled in the past three years to £100bn, research suggests.
The deficit would be equivalent to about 7% of the UK's annual economic output, and compares with a shortfall of £42bn three years ago.
The report was conducted by John Ralfe, an independent pension consultant.
However, some accountants and trade unions say this estimate of the deficit is exaggerated.
The Local Government Pension Scheme in England has four million members including 1.7 million current workers.
It is a single scheme, although it is funded and administered by 81 regional pension funds.
Changes to the scheme, along with all the other big public sector pension schemes, are being considered by an independent commission under Lord Hutton, the former Labour pensions minister.
His initial recommendation, made in October, was that members of public sector pension schemes should be asked to pay higher contributions.
According to Mr Ralfe, the value of assets in the local government scheme has risen by just 8% in the past three years, to £132bn, whereas liabilities have soared 41% to £232bn.
This increase in liabilities has been caused by a number of factors, including a rise in the value of benefits for local authority staff.
However, his principal assertion is that the separate funds should assume that their future investment returns will be in line with the return on corporate bonds, and thus lower than they are currently assuming.
This is an accounting standard known as FRS17, which is used by private companies to value the liabilities of their pension schemes in company accounts.
Mr Ralfe argues that this is a much more realistic method than the one used by scheme actuaries, which allows them to assume a higher return based on the assets actually owned by the schemes.
The effect of using FRS17 is that it inflates the stock of assets which schemes need to hold to pay their pensioners in the future and thus increases their deficits.
The Chartered Institute of Public Finance and Accountancy downplayed the analysis.
The head of the institute's pension panel, Bob Summers, said a better guide would be the scheme's forthcoming three-year actuarial valuation.
"There is the potential for some very highly misleading information," he said.
"What happens every three years - and we're just in the process of that process being completed - is that an independently appointed actuary looks at each and every fund.
"It is a fund backed by investments which means we can take a long-term view," Mr Summers added.
However, the UK government said it was clear that the local government pension scheme was too expensive.
"Town hall pensions are now costing over £300 a year to every household paying council tax," said the Communities Secretary, Eric Pickles.
"This is why action needs to be taken to reduce the massive and unsustainable cost of state sector pensions, creating a system which is fair both to taxpayers and council workers, especially the low paid."
But the GMB trade union, which has many members in local government, denied there was any pension crisis.
"In effect Mr Ralfe is [taking] the equivalent of taking a snapshot of your personal finances part way through a mortgage," said Brian Strutton, the GMB's national secretary for public services.
"It looks like you've got an unaffordable debt but the reality is to look at the long term and whether you can meet that debt."
In future, pensions from the local government pension scheme will be uprated in line with the Consumer Prices Index - a generally lower measure of inflation than the Retail Prices Index which has been used up until now.
Mr Ralfe estimates that will save £20bn, but he believes that taxpayers and scheme members will have to pay more into local authority pensions - some £4bn extra a year.
However, Mr Ralfe says that the official actuarial deficit which the scheme will unveil next spring will be considerably lower, because it is allowed to use much less conservative valuation methods than private sector schemes.
Mike Taylor, the chief executive of the London Pension Fund Authority, said the LGPS would undoubtedly have to be changed.
"I don't think the final-salary scheme can survive. I think that'll be replaced by a career average scheme," he said.
"I would also not be surprised to see reductions in benefits.
"The chancellor has already announced that contributors will pay more and we're likely to see an increase in the retirement age as well," he added.