The Office of Fair Trading is taking enforcement action against payday lenders who are flouting guidance on online advertising in the run-up to Christmas.
In a sweep of a sample of firms it found that some were not checking whether customers could afford to borrow or coming clean about charges for going into arrears.
A number were unclear about the loan terms and failed to explain the contract.
The OFT has not named the firms involved. They could be stripped of their Consumer Credit Licences or simply be forced to change their practices.
The consumer watchdog has started to check advertising on the websites of 50 further firms.
A BBC viewer, Andrew Beattie, complained today that he'd received a text from a payday lender, saying:
"Need extra cash for XMAS? Get cash in your account TODAY. No paperwork or checks."
"Many people may be tempted to follow this up, with no doubt dire consequences," he warns.
In a report sent today to MPs on the Business, Innovations and Skills Select Committee, the OFT says that complaints about the lenders to Consumer Direct rose to 1,535 in the first eleven months of this year, up from 700 in the whole of 2010.
Short-term payday loans have boomed. The market is worth about £2bn a year.
But there are serious concerns about the business practices adopted by some players.
Top of the OFT's list of worries is the misuse of continuous payment authority, which allows a lender to take funds from a borrower's bank account even if the account is overdrawn.
Next comes the rolling over of loans which can result in debts escalating out of control.
But irresponsible advertising and selling tactics have also become a major worry.