Complaints to the Financial Ombudsman Service about payment protection insurance increased year on year by 51%, and accounted for 60% of all new cases in 2011/2012.
Consumers brought 157 716 new PPI-related cases to the ombudsman - the largest number ever received in a year by the FoS about a single financial product.
In its annual report, the FoS said that 150 new staff have joined since January to deal with the volume of new cases.
From April 2012 it has also charged businesses a supplementary case fee of £350 for each PPI mis-selling case referred to the ombudsman, chargeable when businesses have more than 25 of these cases a year.
The ombusdman added: "However, during the year we have again seen some claims management companies taking a disappointingly lax approach to completing the questionnaire on behalf of consumers.
"Where a form is completed inaccurately - or not at all - we may not be able to progress the complaint.
"We have made our position on this very clear to the claims management companies in question, and we have required a number of them to withdraw their complaints and to re-complete the questionnaires again in full."
The FoS added: "We have also continued to see a significant number of cases where, after investigation, it emerged that no PPI policy had ever been in place.
"It is essential that businesses and claims management companies make every effort to check this before the matter is referred to the ombudsman service."
Some 10% of the 264 375 total new cases received in the year ending 31 March 2012 related to insurance, excluding PPI.
