Scottish Widows, Norwich Union, Friends Provident, Family Assurance and Guardian Financial Services are among life insurers being forced to live up to promises that annuity incomes would not fall below a guaranteed percentage.
The guarantees, mostly made in the 1970s and 1980s, typically promised customers an income worth at least 7.5 per cent of the fund they had saved up. They were made in an effort to attract customers and allow them to plan financially.
But the promises now threaten to lose insurers' substantial sums of money because they can no longer find assets yielding enough to back the promises.
Sun Life of Canada last week said it was setting aside pounds 114m to cover annuity guarantees given by its UK wing, and by Confederation Life UK which it took over in 1994. Guardian Financial Services indicated it has also reserved up to pounds 20m in respect of the guarantees.
Mark Oldcorn, a life insurance analyst at Moody's, the credit rating agency, said: "The promises were to pay a minimum return on annuities, but the assets backing those annuities are not yielding sufficient income to pay the guaranteed return."
Insurers use 15-year government gilts to back the income payments they made to holders of annuities.
Between 1980 and 1998 these typically yielded more than 10 per cent, making the guarantees seem safe.
But gilt yields - which track long-term interest rates - are now at a 20-year low, causing the guarantees to come unexpectedly into play.
Peter Magliocco, of annuity specialist the Annuity Bureau, said: "The companies writing these contracts never thought they would have to pay the rates they were offering because annuity rates in the market were so much higher.
"When they set them up, they thought they would never have to pay them."