Less than half of pensioners retiring between 2016 and 2020 are set to receive the full-state pension, according to official figure obtained by Hargreaves Lansdown.
From April 2016, the introduction of the new single-tier pension, aimed at simplifying the system, will see retirees receive a single payment of £148.50.
However, a Freedom of Information request submitted by Hargreaves Lansdown, revealed that just 45% of those retiring in those five years will get the full-state pension amount.
The figures suggest that nearly 2 million pensioners will get less than they might have been led to expect by the government, the firm said.
Hargreaves Lansdown explained that this was due to the majority of people's private or workplace pension terms, which meant they were likely to have been contracted out of the state second pension - for defined benefit pension schemes - which is being included into the new flat-rate state pension.
For others, there is a gap in National Insurance contributions.
'The new state pension will ultimately be a simpler and fairer system,' Tom McPhail, head of pensions research at the firm said.
'However in the short term it will be complicated and many people are likely to get less than they may expect.'
He said it was imperative that people received a proper state pension forecast, as it was likely that they could spend their all their private pension savings, following the new freedoms coming into affect this April.
'Without this [pension forecast], they could get a nasty shock when they do reach state pension age.'