Only 45% of the 3.5 million workers due to retire between 2016 and 2020 will receive the full state pension of about £150 a week to be introduced in 2016.
The figure was obtained by the investment business Hargreaves Lansdowne under a Freedom of Information request. About a third of those who retire will receive no more than £133.56 a week.
A government spokesman said that no person would be worse off under the 2016 scheme than they would be under current arrangements.