The Financial Services Authority (FSA) received more than two million complaints in the second half of 2011 - a rise of 21% from the first six months of the year.
The majority of these were to do with payment protection insurance (PPI) and complaints in this category were up 85% from July to December 2011.
In April 2011, banks were ordered to start working through a huge backlog of PPI cases by the FSA.
The insurance is taken out to cover repayments of loans should you fall ill or find yourself out of work. But many people were mis-sold this policy and so far £2.1 billion has been paid out in compensation.
Complaints about banks were down 13% annually and at their lowest levels since 2006 but those regarding credit cards and unregulated loans were up in the second half of the year.
Complaints upheld in favour of consumers were up 51% over the year and this was mainly in response to PPI.
"The increase in PPI complaints further demonstrates just how widespread PPI mis-selling was. We now need to see the banks take action and deal with these complaints quickly and efficiently and the FSA must take action against anyone dragging their feet settling complaints," says Richard Lloyd, executive director of Which?
"We need the new financial regulator, the Financial Conduct Authority, to be a watchdog not a lapdog. It must stand up for consumers and stand up to the banks," he adds.