There were around 4.13 million gripes made to 3,161 firms but 98% of the grievances were with just 235 firms, according to data from the Financial Conduct Authority (FCA).
Disruption to retail banking services, including the IT meltdown suffered by TSB, in part accounted for the increase in complaints to certain firms, the FCA said.
Mortgage complaints made up 3% of the total, behind payment protection insurance (PPI), current accounts, credit cards and car insurance.
However, home finance products received the most complaints as a proportion of the total number of products, with almost 10 issues for every 1,000 policies sold – compared to almost five for banking and credit cards.
Marks and Spencer Bank received the most complaints proportionally with 367.7 complaints for every 1,000 policies sold, followed by Blemain Finance and Al Rayan with 66.2 and 55.1 respectively.
There was a slight fall in the upheld rate for mortgage complaints at 58% in the first half of 2018, compared to 60% in the six months prior.
Christopher Woolard, executive director of strategy and competition said: “Firms need to be doing all that they can to reduce complaints.
“It is clear that firms need to look at the cause of the rise in complaints and address these issues to prevent further increases.
“It should be a priority for firms to ensure good consumer outcomes are achieved and they should be making sure that they are taking the right steps to treat customers fairly.”
An M&S Bank spokeswoman said: “We have received just 25 mortgage-related complaints since launching our first mortgage range earlier this year.
“Delivering the exceptional levels of service our customers have come to know and expect from M&S Bank is our absolute priority and we have acted on the insight from all 25 cases.”