Consumer groups have protested about PPIs for two years.
The Office of Fair Trading asked for the investigation.
The commission may impose temporary price limits on policies until prices come down.
HSBC and some other finance companies anticipated the report’s findings, and stopped offering PPIs before it was published.
The commission allowed responses until June 30.
Peter Davis, commission deputy chairman, said consumers were getting a raw deal.
Selling PPIs as an add-on meant distributors escaped competition pressure on price and quality and there was little direct advertising.