Santander overtook Bank of Scotland to become the most complained about provider of home finance products in the second half of 2011.
The lender received 10,874 complaints in the second half of 2011, up 21% from the 8,961 complaints in the first half of the year.
Bank of Scotland, which includes Halifax, received 10,034 complaints, down 17% on the first half of 2011 when it received 12,080.
But Lloyds Banking Group still overtakes Santander with 12,724 when the 2,690 complaints about its subsidiary Cheltenham & Gloucester are taken into account.
Home finance products include regulated and unregulated mortgages, equity release and impaired credit mortgages.
Overall, the number of complaints about home finance products was static at 69,088, compared with 67,537 in the first half of 2011.
A spokeswoman for Santander says: “While we are always looking to improve, it should be noted that our share of total complaints is lower than you would expect for an organisation with our market share.”
Meanwhile, the number of complaints about mortgage businesses, which includes brokers, lenders and administrators, shot up by 76%.
There were 52,746 complaints about mortgage businesses in the second half of 2011, up from 29,917 in the first half.
In total, mortgage firms paid out £44.4m in redress in the second half of the year, double the £22.2m of the first half.
The most common cause for complaint related to advising, selling and arranging products.
But a spokesman for the FSA says that the complaints relating to mortgage businesses may not necessarily stem from complaints about products.
He says that a firm may list itself as a mortgage business if its primary business is mortgages, but the complaints may also be due to other products such as payment protection insurance.