The revised estimate came as companies involved in mis-selling pensions were ordered to review another 1.8 million cases.
Three years ago an investigation was launched into the 500,000 most urgent mis-selling cases which has already cost more than £4bn in compensation.
Financial watchdogs say the extended review will focus on younger investors who are still more than 15 years away from retirement.
The Financial Services Authority and the Personal Investment Authority are to launch the second phase of their pensions review, aimed at people wrongly sold personal pensions when they would have been better off remaining in, or joining, a company scheme.
The review has so far concentrated on "priority cases" involving people who had retired, were close to retirement, or had died.
The FSA and PIA said the second phase of the investigation would look at cases involving younger investors who were still 15 years or more away from retirement.
An estimated 1.8 million investors fall into this category, and between them they face losses totalling £6.6bn.
Compensating them could cost between £3.3bn and £5.8bn, said the financial watchdogs.
Pensions pitfalls
(13 Mar 98 | Business)
Pressure mounts on pension firms
(12 Mar 98 | UK)
Record fine for Britannic over pensions scandal
(03 Mar 98 | Business)
Record fine for insurer over pensions review
(28 Jan 98 | Business)
New powers to discipline 'pension laggards'
(19 Nov 97 | Business)
The man from the Pru says sorry
(17 Nov 97 | Business)
Pensions Acts 1995
Pensions Compensation Review Board Regulations 1997
HM Treasury
Occupational Pensions Regulatory Authority
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