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Councils in Wales face a "ticking time tomb" of pensions deficits measured in billions, according to the lobbying organisation the Taxpayers' Alliance.

It found Cardiff had the largest deficit in Wales in 2010 at £494m, or £1,450 per head of population.

Merthyr Tydfil had the highest deficit per head of population in the entire UK at £2,268 per head.

Councils called the report "spurious" and said they had taken steps to ensure staff pensions were affordable.

The Taxpayers Alliance looked at the pension schemes of all 101 local government pension schemes in England, Scotland, Wales and Northern Ireland.

It said it obtained the information from local authorities' annual statements of accounts in 2010-11 and used actuarial estimates for assets held by councils and their liabilities to calculate their overall funding situation.

The organisation said local government final salary schemes - where entitlements are calculated on the basis of people's salaries at retirement - were "inflexible, too expensive" and needed urgent reform.

It said councils should not take "false comfort" in the improvement in the stock market.

Robert Oxley, of the Taxpayers Alliance, said: "It's like ignoring your mortgage. Something does have to be done to bring down this deficit.

"We can't pretend there aren't issues of reform."

But Dominic Macaskill, head of local government for the union Unison in Wales, said: "The assumptions are predicated on the public sector closing down and nobody ever paying in to the scheme again and all the liabilities coming at once."

The Welsh Local Government Association added: "The picture painted in the report is based on a spurious way of gauging the viability of a pension scheme, and does not reflect the changes that have already been made to the scheme to help secure its long term viability."

UK government ministers and unions are currently negotiating over long-term changes to the scheme's structure, with the government proposing to increase the amount workers contribute, raise the retirement age and make changes to the accrual rate - the rate at which annual benefits are earned.