Controversial changes which will result in people having to work longer before they can collect their pensions are the result of a more affluent and healthy society, Nick Clegg has said.
The Deputy Prime Minister defended the changes, which will be set out in George Osborne's Autumn Statement, although he acknowledged it was not an "easy concept".
People now in their 40s will not now get the state pension until they are 68, while those in their 30s will have to wait another year until they are 69, sources revealed ahead of the Chancellor's Commons statement.
Mr Clegg told LBC 97.3 that the "rule of thumb" for a long period had been that people would be able to draw their pension for around a third of their life.
"If that's your rule of thumb, that sense of entitlement that around a third of your adult life is spent in retirement is to be retained, then of course it needs to keep up with the fact that people are living longer and longer and longer," he said.
"That, of course, clearly means that the point at which you retire shifts and that's what the Chancellor is confirming in the Autumn Statement today."
Mr Clegg added: "I'm not pretending it's an easy concept to accept but it's part of what happens as societies become more affluent and as people become healthier and live much, much longer.
"We need to make sure that the rules of retirement, which were first set decades ago, keep up with those demographic changes."
News of the move came after Mr Osborne announced a further £3 billion Whitehall spending squeeze over the next three years.
It came despite strong expectations that the Chancellor will announce a sharp upgrade in the Office for Budget Responsibility's (OBR) official forecast for economic growth.