The Financial Ombudsman Service (FOS) has said the number of complaints it has received about investments has increased by a third this financial year.
The number of cases relating to investments - including pensions and mortgage endowments - has increased by 31% compared with 2011/2012.
By the end of the year the FOS expects to have received around 18,400 new cases, against a planning assumption of 14,100.
Continued market volatility has contributed to this increase, it said. Falls in the value of some investments has prompted some consumers to complain that what they bought did not match the description they were given.
These cases tend to be complex - and can take more time to assess and resolve than more straightforward cases, FOS said.
The service has also seen an increase of more than a third in the number of complaints about mortgage endowment policies - after numbers had been falling for several years.
These cases largely involved consumers concerned about the shortfall that is typically seen in these policies when they mature.
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