The company has surveyed its own personal pension policyholders - there are about 340,000 - and found that the average contribution is pounds 78 a month and fewer than 20 per cent of policyholders are increasing their contributions on a regular basis.
According to calculations by NPI these people are on course for a meagre living in old age. A 40-year-old man planning to retire at 60 and contributing pounds 100 a month to a personal pension could expect to receive a monthly pension of pounds 104 (after allowing for 5.5 per cent inflation) if his invested contributions grow at 8.5 per cent, but inflation during the life of his plan is 5.5 per cent a year.
He would also receive a lump sum of pounds 4,540 (after inflation).
If he chose not to take this, he could increase his monthly pension, but only to pounds 138.
These figures all assume expenses at the standard amount stipulated by the Life Assurance and Unit Trust Regulatory Organisation (Lautro) for quotations to prospective pension purchasers. Some companies will have higher expenses.
Laurie Edmans, assistant general manager (marketing) at NPI, says many pensions sold in the last few years have quoted likely returns on the basis of a much higher level of investment growth than 8.5 per cent, minus inflation.
One of the standard figures allowed by Lautro for quotations has been 13 per cent.
Mr Edmans says this is far too optimistic a level of return to expect over the next few years.
'We believe anyone with a personal pension should ask their pension company or independent adviser what their pension will be worth in real terms.'
He says that if the 40-year-old paying pounds 100 a month was to increase his monthly contributions by 7.5 per cent a year to retirement, he could expect to draw a pension of pounds 189 a month, (after 5.5 per cent inflation) plus a lump sum of pounds 8,240, after inflation.
NPI will be writing to policyholders next spring explaining what they can expect to get for their present contributions, after inflation, and pointing out that their policies have the facility to increase premiums each year automatically.