The number of complaints about residential first charge mortgages increased by around a third quarter on quarter, according to FOS data.
Enquiries surged by more than half from 799 to 1,632 in the same period.
The uphold rate on mortgage complaints was 18 per cent, compared to 32 per cent across all FOS cases.
Overall there were 68,735 complaints in the quarter, up by a fifth on both the previous three months and the same period in the previous year.
The Covid crisis helped contribute to the rise in issues, with travel insurance, business protection insurance and commercial lending the products most affected by the pandemic.
There was also an increase in issues from the number of people who borrowed money and the felt the debt was unaffordable, the FOS said.
Payment Protection Insurance (PPI) remained the most complained about product but accounts for a declining amount of the ombudsman’s case load.