Many could lose average payments of about pounds 10,000, needed to restore their pensions to levels reached before the mis-selling. When they retire, the value of their pensions will be cut accordingly.
Up to half of policyholders - many of them likely to have suffered the heaviest losses because of mis-selling - have not replied to their insurers' letters.
Although there is no time limit for replies, many insurers believe that once public interest in the pension-selling scandal wanes, response rates may remain low.
Last year, several insurance company executives privately said the easiest way to cut an expected pounds 3bn compensation bill would be to do only the strict minimum when contacting clients.
Kate Scribbins, head of the Consumers' Association money group, said: "This is a low response rate. We have had a concern throughout this process that the onus is on the consumer to react to the companies.
"This is only going to work if there is a high-profile campaign to inform people of what they should do.
"If this does not work and the response rate remains low, the Personal Investments Authority [the regulator] must be prepared to set in place automatic reviews of personal pensions."
The insurance company mailshots follow a report last year by the Securities and Investments Board, the City's most senior regulator. The report says up to 1.5 million people may have been wrongly advised to set up personal pensions and transfer their money out of occupational schemes.
The SIB and other watchdogs set up a system for people to receive compensation. The first stage involves a questionnaire designed to assess whether mis- selling had taken place and the level of any loss.
In the past few months, insurers have written to more than 1 million of their policyholders at least once. About 350,000 of the most urgent cases, often elderly people who were advised to switch their funds out of company schemes and into private ones, must be considered first. Industry estimates suggest compensating these so-called "opt-outs" could cost an average of pounds 10,000 each.
Legal & General has mailed tens of thousands of clients three times and also tries to contact them by telephone. Despite its efforts, the response rate has barely reached 50 per cent.
Steve Bee, pensions manager at Prudential, Britain's largest insurer, said his company had recently begun contacting 500,000 of its clients at the rate of 30,000 a week.Reuse content