Less than half of pensioners retiring between 2016 and 2020 are set to receive the full amount of the new flat-rate state pension.
Around 3.5 million workers will reach state pension age during the five-year period but just 45% will be entitled to claim the full amount of no less than £148.40 a week – meaning nearly two million people will get less than they might have expected to receive.
A person's National Insurance (NI) record is used to calculate how much they are entitled to receive, with workers usually needing to have accumulated at least 10 qualifying years to claim any part of the state pension.
According to Hargreaves Lansdown, which uncovered the figures, the majority of people who will not qualify for the full pension are those with interrupted NI contributions, such as mothers and the self-employed.
Some 30% of pensioners will get less than 90% of the new entitlement, meaning they will be entitled to no more than £133.45 a week; while 10% of pensioners will get less than £118.72 a week.
Less than you expect
Tom McPhail, head of pensions research at Hargreaves Lansdown, said: "The new state pension will ultimately be a simpler and fairer system. However, in the short term it will be complicated and many people are likely to get less than they may expect.
"With the new pension freedoms meaning that they will be free to spend all their private pension savings, it is imperative that they receive a proper state pension forecast. Without this, they could get a nasty shock when they do reach state pension age."
The new state pension will come into being on 6 April 2016 if you are a man born on or after 6 April 1951 or a woman born on after 6 April 1953, while new pension freedoms announced by the Government in last year's budget which will see people able to access their private pension before they reach pension age, will come into affect from this April.
The actual amounts for the new flat-rate state pension will be confirmed in the autumn.
Meanwhile, a report from Saga has found that an extra £1.5 billion will be taken out from pensions once withdrawal rules are relaxed in April. The figure is based on a government estimate that the 320,000 people who retire with a defined contribution pension each year will take out 30% of their funds instead of the current maximum allowed of 25%.