Around two million people will get less than the supposed "flat-rate" payment during the first five years of the scheme.
The revelations, contained in an email seen by The Telegraph, will give a "nasty shock" to many workers in their late fifties and early sixties, experts predicted.
Ministers had promised to replace today's means-tested state pension, which consists of a basic payment and top ups linked to career earnings – with a single payment of at least £148.40 in today's money.
Workers were told that everyone who reached pensionable age after April 2016 would see their national insurance contributions count towards a "single tier" amount.
But the first official calculations for the proposed payments show just 45 per cent of people reaching pensionable age between 2016 and 2020 will receive £148.40 or more.
Tom McPhail of pension company Hargreaves Lansdown said: "The new state pension will ultimately be a simpler and fairer system, but 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."

