Six life insurance firms are being investigated by the City regulator after it raised concerns that some long-standing customers may not have been told when charges were applied.
Abbey Life, Countrywide, Old Mutual, Police Mutual, Prudential and Scottish Widows are being investigated to determine whether they failed to meet standards and see if further action is needed, the Financial Conduct Authority (FCA) said.
But the regulator said the probe into the six firms does not mean there will necessarily be any disciplinary action or penalties - and it has not ruled in or out any potential redress payouts to customers.
In the meantime, customers who feel they have been poorly treated can still complain to their firm, and to the financial ombudsman.
The probe comes after the regulator carried out a review, looking at 11 firms in total, holding £153 billion in "closed book" assets collectively, across 9.4 million customers. Closed book customers have been with their firm for some time and their products are no longer being actively marketed.
The FCA found evidence of poor practice - with particular concerns around how information was communicated about exit fees as well as paid-up fees - where customers stop paying premiums but are still in the policy.
The regulator is concerned that where exit or paid-up charges were applied, some customers may not have been told at the time.
It is also proposing to cap or remove these fees under a voluntary agreement across the industry. It said it expects life insurance firms outside its sample to make any changes necessary.
The FCA emphasised that no conclusion has been reached on whether there have been any breaches of regulatory requirements.
The probe is likely to take several months - but in the meantime if customers are unhappy, they can still follow the normal procedure of complaining to the firm, and if they are dissatisfied with the outcome they can take their complaint to the Financial Ombudsman Service (FOS).
A spokeswoman for the FOS said: "If you're unhappy with the way you've been treated, get in touch - our approach hasn't changed."
The FCA's review focused on how customers are being treated now, not how the policies were sold in the first place.
The impact exit fees and paid-up charges can have on returns can be "significant" and may be a barrier to customers shopping around, it said.
For all six firms being investigated, the FCA will focus on the disclosure of exit and paid-up charges after December 2008.
The regulator said it is not possible at this stage to draw conclusions on the reasons for some firms' practices, or to say whether customers suffered detriment.
Tracey McDermott, acting chief executive of the FCA, said: "We expect all firms with closed-book customers to take into account the findings we have published today and ensure they are treating their closed-book customers fairly.
"The practices at some firms appear to have been poor. We have particular concerns regarding how some firms communicated with their customers about exit and/or paid-up charges.
"We are now doing further work to understand the reasons for these practices, whether customers may have suffered detriment as a result and if so, how widespread these issues are."
Hugh Savill, director of regulation at the Association of British Insurers (ABI), said: "This is an important report the findings of which we will study in detail.
"It should be recognised that products analysed in the review bear little resemblance to the long-term savings market today, which continues to modernise and deliver value for money products with lower charges in the era of auto enrolment and pension freedoms."