The Financial Conduct Authority (FCA) is writing to clients of advisers who reported a below average opt-in rate for its Arch Cru redress scheme - to ask if they believe they had been properly informed.
It wants to know if firms sent the clients the "appropriate communications" about the scheme.
Under the redress scheme, which was announced in December 2012, firms that advised on investments in the CF Arch cru Investment and Diversified funds have been required to contact all their clients asking if they want their case reviewed.
Clients who opt for a case review and receive redress will be put "back into the position they would have been in had they received suitable advice".
On average, the FCA says 48% of advised Arch Cru investors - equating to more than 3,300 individuals - had requested a review, but it noted some firms reported far lower opt-in rates - as little as 20% in some cases.
As reported by IFAonline.co.uk in August, the FCA is writing to those firms for an explanation.
But it is understood it will be writing to clients too, to assess whether firms had sent them the "appropriate" communications.
The redress scheme came into force this April and advisers had until 29 July to report back to the FCA.
However, the Financial Services Compensation Scheme said that investors who had not opted in to the scheme before the deadline could "still complain to the advising firm at a later date, although the adviser would not then be required to consider the complaint under the requirements of the CRS".
The CRS was drawn up to accompany the £54m payment scheme that was already in place to compensate investors in the Arch Cru funds.
The CF Arch Cru funds were suspended by its authorised corporate director Capita Financial Managers in March 2009.
The FSA later concluded that there was evidence of widespread mis-selling by firms who failed to assess the funds as high risk despite the fact that the funds were typically invested in non-mainstream assets such as private equity, private finance and commodities.
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