The regulator said in its interim report last autumn that formal investigations have been launched into several firms over their debt collection methods.
Charities have reported rocketing numbers of complaints about payday lenders from borrowers. The Money Advice Trust (MAT) recently said that complaints about payday loans have doubled year-on-year to reach a record of 20,000 across 2012.
The charity warned that "something is drastically wrong" with the way that expensive loans are being dished out to people who cannot afford them, with lenders often rolling over loans.
New regulator the Financial Conduct Authority (FCA), which will oversee the consumer credit market from next year, has committed to prioritise tighter rules on payday lending that could come into effect from April 2014.
The FCA's rules will be binding and if they are broken the regulator will have tough enforcement powers including imposing unlimited fines and the ability to claw consumers' money back.