Pensioners who have already retired have expressed their anger that they will not benefit from the flat-rate pension when it is introduced in four years time.

UK adults who retire in 2016 and have made sufficient National Insurance (NI) contributions will be eligible for the new flat-rate pension of £140 a week.

However, existing pensioners - as well as those who are due to retire between now and 2016 - will stay in the existing state pension scheme and are likely to be worse off than their younger counterparts.

Ros Altmann, the director general of Saga, the over-50s' group, revealed that the organisation has received many letters from pensioners complaining about the issue.

She told the Telegraph: "I understand that existing pensioners getting less than £140 may be unhappy. I would call on the government to consider extending the flat-rate system to those who have already retired."

The Pensions Policy Institute has estimated that extending the policy would cost an additional £5.8 billion a year.