The other firms involved were Abbey Life, Scottish Widows, Countrywide and Police Mutual, the Financial Conduct Authority (FCA) said.
The financial watchdog has been monitoring whether insurers have treated customers locked into pension and other savings plans fairly compared with new customers.
As part of this assessment, the FCA sampled a number of documents sent to customers who had requested either to surrender or transfer their policies. The FCA said the six firms may have failed to inform customers of charges they were likely to incur.
“The practices at some firms appear to have been poor,” acting FCA Chief Executive Tracey McDermott said in a statement on Thursday.
An investigation could lead to the FCA imposing fines or demanding insurers pay compensation to policy holders.
“The regulator isn’t pulling any punches, and looks set to take action across the life industry based on these findings,” said Matt Browne, conduct risk director at PwC’s insurance practice.
“The review is going to have a big impact on life assurers.”
Insurers are already under regulatory strain due to a number of recent changes including major pensions reforms, which could include a cap on charges.
Old Mutual, Scottish Widows and Countrywide Assured-owner Chesnara (CSN.L) said they would cooperate fully with the regulator. Shares in Chesnara were down 5 percent.
Deutsche Bank (DBKGn.DE), which owns Abbey Life, declined to comment, while Prudential and Police Mutual did not immediately respond to requests to do so.
A briefing to some media outlets in 2014 by the FCA that an initial review was planned prompted a slump in insurer stocks and led to an inquiry and the departure of the watchdog’s director of supervision.
It is also as having contributed to the early exit of the FCA’s former chief executive Martin Wheatley.
Reporting by Richa Naidu and Carolyn Cohn; Editing by Sinead Cruise and Keith Weir