Watson Wyatt calculated the figure using FRS 17 which is the new accounting standard which all listed companies must use from 1 January 2005.

It is 50% higher than government estimates, and dwarfs the national debt which currently stands at £400bn.

Public sector pension liabilities are now growing at a rate of £46bn a year, enough to add 14p to income tax, the Daily Telegraph reported.

Last week actuaries Lane Clark & Peacock calculated that the deficits of FTSE100 pension schemes dropped last year from £55bn to £42bn.

FRS 17 requires businesses to show pension fund assets and liabilities on the balance sheet for the first time.