In the nine months between April and December 2016 the FOS received 1,385 complaints related to Sipps, compared to 1,288 Sipp complaints in the same period in 2015.
The ombudsman’s complaints data listed Sipps as the joint highest product, alongside pay day loans, for the percentage of upheld complaints over the same period.
As a result, complaints upheld against Sipps rose steadily from 52% in the nine months to December 2015 to 56% in the same nine months last year.
The figures come just months after the Financial Services Compensation Scheme (FSCS) warned it would have to impose an interim levy of £36m (€41.7m, $44.9m) on life and pension advisers over the increasing number of claims related to Sipps.
Last July, the lifeboat fund revealed that payouts over Sipps claims have more than doubled due to an increase in high risk Sipp investment claims.
The FSCS also raised its expected levy for life and pension advisers to £171m for the 2017/18 financial year as it predicted the trend of complex Sipp complaint numbers to rise.