Millions of people could be required to work until they are 68 before claiming the state pension under plans to bring forward the increase in the retirement age by at least seven years.
The Office for Budget Responsibility said it assumes the state pension age will rise to 68 between 2037 and 2039 after the Treasury confirmed this was the government's "current policy position", rather than the timetable set out in existing legislation.
The move would affect around five million people currently aged between 49 and 55 and is estimated to save the Treasury about £6billion a year from 2037.
Ministers insist no final decision has been taken while a government review is under way.
However, pensions minister Torsten Bell told The Times: “We want to make sure that we have a sustainable state pension for the longer term.”
Former pensions minister Sir Steve Webb urged ministers to provide clarity, saying: “That means around five million people will lose around £12,500 that they might otherwise have been entitled to. Ministers are going to have to be clear about this soon.”