Governments and the financial sector have consistently underestimated how quickly average lifespans will rise, often understating the increase by as much as three years, the IMF said.
Last year the Institute and Faculty of Actuaries warned COVER that period life expectancy, commonly quoted as the average number of additional years a person can be expected to live for, is not realistic as it assumes mortality rates are frozen to that time period.
For Britain, the IMF calculated that, on the "not unreasonable" assumption that the cost would fall on taxpayers, the country's public debt would rise from 76% of GDP to as much as 135%.
Addressing the debtly hallows Reports calculate that additional cost would be about £750bn.
The IMF said the extra costs would come from the state pension and public sector pensions, while some would also come from the state having to rescue failed private sector schemes.
The latest estimates from the Office for National Statistics suggest that boys born in 2010 will live an average of 78.2 years, while girls will live an estimated 82.3 years.
But the IMF highlighted research by the ONS which suggested that such forecasts suffer "widespread" errors.
Projections of future life expectancy are "consistently too low in each successive forecast, and errors were generally large", it said.
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