The Lloyds-owned provider attracted 705 complaints, which was significantly higher than the next in line, Santander, with 412.
The third most-complained about lender was Barclays at 372 and next, both having received 201 complaints, Lloyds Bank and NatWest.
The total number of complaints made to the FOS in H1 2019 was 161,390, of which 2.9 per cent, or 4,693, related to mortgages and home finance. The FOS upheld 29 per cent of complaints overall, rising to 39 per cent when excluding payment protection insurance (PPI).
Mortgages and home finance
In mortgages and home finance, the top 10 most-complained about lenders accounted for 57 per cent of all complaints, or 2,674 cases.
The total number of mortgages and home finance complaints was 4,693.
The lenders placed in the lower half of the top 10 were Nationwide Building Society (208), TSB (168), HSBC (163), Northern Rock Asset Management (121) and Royal Bank of Scotland Plc (105).
Complaints are falling
The FOS’s data showed that the number of complaints has been falling over the past 18 months.
The total number of complaints dropped from 190,307 in the second half of 2018 and from 201,600 in H1 2018.
The number of mortgages and home finance complaints has followed a similar trajectory, falling from 5,374 in the second half of 2018 and from 5,418 in H1 2018.