According to The Guardian on Saturday, The National Institute of Economic and Social Research said the saving outlined by Osborne at the Conservative party conference in Manchester last week would take five years longer than estimated and would fall £3bn short.
NIESR said Osborne’s team had misread a paper written by the thinktank earlier this year.
Osborne said he would save £13bn by raising the state pension age from 65 to 66 in 2016 for men and 2020 for women.
Liberal Democrat Treasury spokesman Lord Oakeshott said: “This saga of incompetence shoots to pieces his claims to be a responsible Chancellor.”
A spokesman for Osborne said the £13bn of savings included inflationary rises between 2009 and 2020.
He said: “We were clear that the £13bn figure was based on a projection of the cost of money in 2020. We accept the gain in today’s money is only £10bn.”