Lord Turner's proposals to reform the state pension system will add next to nothing to public expenditure be-tween now and 2020 - the longest period Gordon Brown could conceivably hope to be prime minister - the Pensions Commission will make clear next week.

By 2040, however, the commission will say that its proposals would add between 1.3 per cent and 1.8 per cent of gross domestic product to pension spending compared with today's figure: the equivalent in today's money of an extra £14bn to £22bn on top of the £60bn currently being spent.