Britain\'s ageing population is threatening a pensions time bomb that could cost as much as 750 billion pounds, the International Monetary Fund has warned. The IMF said that even a slightly faster than expected increase in life expectancy could impose a huge new financial burden on Western economies such as Britain, according to UK\'s The Daily Telegraph. For Britain, the IMF calculated that on the \"not unreasonable\" assumption that the entire cost would fall on taxpayers, the country\'s public debt would rise from 76% of gross domestic product to as much as 135%. In today\'s money, that additional cost would be about 750?billion pounds.