The Office of Fair Trading has told Wonga.com to improve aspects of its debt collection practices.
The OFT found that the company had wrongly suggested to some customers working in the financial services industry that they may have committed fraud.
It stated that customers with jobs in the public or financial sectors should not find themselves in debt, and that this was stated in their terms of employment.
The letters and emails were sent to customers who had claimed money back from Wonga.com by asking their card providers to reverse a payment made to the company. They were also sent to some clients who had entered into debt management plans.
Wonga.com says it plans to appeal the decision. It says although it believes it had grounds to suspect dishonest conduct by the customers to whom letters were sent, they were all sent on isolated occasions more than 18 months ago.
Wonga.com says it has put in place procedures to ensure these communications cannot be repeated and that it has provided assurances to the OFT to that effect.