Towry and Norwich & Peterborough Building Society each had 92 per cent of complaints upheld against them by the Financial Ombudsman Service during the first half of the year.
Complaints data published by the FOS today for the complaints made against individual businesses between January 1 and June 30 reveal that the level of complaints upheld against Towry and N&P is among the highest out of all financial services firms.
The average proportion of complaints upheld across all firms was 47 per cent.
Some 140 new complaints were made against Towry during the first six months of the year, 138 of which were investment complaints.
N&P received 394 new complaints, 351 of which were related to investments.
The building society was fined £1.4m by the FSA in April for failing to give customers suitable advice in relation to the sale of Keydata products.
AWD Chase de Vere saw 70 per cent of complaints upheld, while Aegon-owned distribution business Positive Solutions had 63 per cent of complaints against it upheld.
Sesame had 56 per cent of complaints against it upheld and Openwork had 43 per cent of complaints upheld.
In terms of the total number of complaints against each firm, AWD received 61 new complaints against it, 41 of which related to investment complaints.
Positive Solutions had 41 new complaints, 19 relating to investments and 12 relating to life and pensions and decumulation business.
Sesame received 76 new complaints, of which 29 related to life and pensions and decumulation business, 17 related to mortgages and home finance, while 16 related to payment protection insurance.
Openwork had 57 new complaints against it, of which 23 were related to mortgages and home finance, 10 about PPI and 9 about investments.