New rules are being introduced by the City watchdog to stop endowment holders from losing out on compensation.
70% of endowment policies are set to fall short of their target
From 1 June, people facing shortfalls on endowment loans must be told there are time limits on complaints.
It is thought many homeowners are unaware there are restrictions on complaints as, until now, insurers did not have to warn them.
Policyholders have three years from their first "red" projection letter - the highest warning - to complain.
Red letter day
Under the new rules, someone who receives a red letter from a firm that imposes time-bars must be told that there is a three-year limit on complaints to the Financial Ombudsman Service.
In addition, people who have received red letters must also be warned at least six months before they would be time-barred from making a complaint.
As an interim measure, people whose complaints will be time-barred between 1 June and 30 November will be given a two month "extension" in which they can complain to the ombudsman.
The move was welcomed by the Consumers' Association, but it believes many will have already missed out.
"Our concern now is for those people who have been told by companies that they are out of time," a spokesman for the Consumers' Association said.
"We encourage these consumers to take their complaint to the Financial Services Ombudsman who deals with these cases individually."
By 31 March this year, 452,201 complaints had been made about endowment mis-selling, an increase of 71,406 since the start of the year.