Around two million retiring pensioners will miss out on getting the full new state pension, according to official figures.
A Freedom of Information request, submitted by financial services firm Hargreaves Lansdown, found that just 45% of those retiring between 2016 and 2020 are set to receive at least £148.40 ($224.43, €189.95) a week.
This means nearly two million pensioners will get less than they might have been led to expect by the government.
The release also revealed that one million pensioners in the first five years are set to receive less than 86% of new state pension.
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," he said.
"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."
With the new pension freedoms due to start in April 2015, all these people will potentially be able to access their private savings before their state pension age.
Hargreaves Lansdown said 30% of pensioners will get less than 90% of the new state pension, meaning that they will be entitled to a state pension income of no more than £133.56 a week.
"Language about people 'missing out' is utterly misleading," a Department for Work and Pensions spokesman said.
"Those retiring when the new State Pension is in place will get at least what they would have got for the National Insurance contributions they have paid in the current system, and many will get more.
"Our historic pension reforms will create a simplicity that hasn't existed in decades, giving people a greater sense of economic security and peace of mind in retirement.
"The new State Pension will tackle inequalities of the past, with women, carers, lower earners and the self-employed set to benefit the most."