More than 600,000 pensioners are believed to have been sold annuity contracts that failed to account for their health in the six-year period under review, the newspaper said.

Most were never told that conditions such as diabetes or high blood pressure could entitle them to an enhanced annuity - providing a fixed income for life - because of their reduced life expectancy.

Around 12 billion pounds' worth of annuities were sold in 2013, according to the Association of British Insurers, but that figure was halved last year after the government said over-55s no longer needed to use their pension pots to buy an annuity.

Major sellers of annuities include FTSE 100 companies Prudential, Aviva, Legal & General and Standard Life.

British insurer Aviva said last year it would compensate 250 customers who should have been sold enhanced annuities because of medical conditions which cut their life expectancy.

The Financial Conduct Authority declined to comment.

(Reporting by Carolyn Cohn; editing by Andrew Roche)

Stocks treated in this article : Aviva plc, Legal & General Group Plc, Prudential plc