Around 70,000 people in their fifties and sixties will miss out entirely on the new state pension between now and 2030, charity Age UK has warned.
Some 50,000 women and 20,000 men do not have the minimum number of qualifying years of National Insurance contributions (NICs), according to the charity's analysis.
Under the new system, coming into effect on April 6, people will need 10 qualifying years to get any new state pension.
This is a change from the existing regime which allows people to receive at least some state pension even with only a few years of NICs.
The full new state pension will be £155.65 per week, and the change applies to women born on or after April 6, 1953 and men born on or after April 6, 1951.
Age UK said it supports the aim of a simpler, fairer state pension, acknowledging it should benefit many people with low lifetime earnings due to low pay and caring responsibilities.
But the charity is concerned that there are many approaching retirement, particularly women, who do not have a full NI record.
Age UK's charity director Caroline Abrahams said: "Big changes are about to come in to simplify the state pension system, yet we know that many people in their fifties and sixties are completely unaware of how they will be affected.
"There is evidence suggesting that some people in this age group are so worried about what their finances will be once they retire that they are reluctant to think about it at all, but our strong advice is to take action now to find out exactly where you stand.
"The reality is that the news will be good for some but disappointing for others, which is why it is so important for everyone approaching retirement to check their state pension age and what they'll receive when they reach it."
A Department for Work and Pensions spokeswoman said: "The new state pension will bring much needed certainty for savers and provide a solid foundation upon which people can build up their retirement savings.
"People who haven't built up enough National Insurance can apply for Pension Credit, which works by topping up their other income and provides additional security in retirement."