Payday lender Wonga has apologised after sending emails implying that one of its loans could "do wonders" for people's credit ratings.
Consumer help website MoneySavingExpert (MSE) warned that the presence of payday loans on people's credit files can cause some lenders to turn people down flat.
It was contacted by an MSE user who received such an email from Wonga last month, which read: "Repaying your Wonga loan on time does wonders for your credit rating!"
Wonga.com apologised for any confusion caused and said that an old version of a customer email had been temporarily used. The lender declined to say how many of the emails were sent but said they were no longer in use.
A good credit history is vital when applying for loans, credit cards or mortgages and lenders generally have toughened their borrowing criteria amid the weak economy, meaning people have higher hurdles to jump to get a deal.
MoneySavingExpert.com money analyst Helen Saxon warned on the consumer website: "Far from benefiting your credit rating, payday loans can actually have the opposite effect.
"Even if they're fully paid off, their presence on your credit file can cause other lenders to turn you down flat, as it could be a sign you can't manage your money."
But Wonga highlighted information from consumer credit information firm Callcredit, which is used by Wonga, which said that in some cases, payday loans can help to improve people's profiles.
Peter Mansfield, managing director of Callcredit said: "In some cases, if used responsibly, payday loans could help to improve the credit profile, especially those with either little credit history or those with a previous history of missed payments and arrears."
He added: "Late and missed payments and large volumes of loans would have a negative impact on a consumer's credit profile.
"However, for many lenders taking and repaying a payday loan will have a neutral or positive impact across large sections of their customer base."
Mr Mansfield said it was difficult to generalise as the interpretation of the details on a consumer's credit file is down to individual lenders.
He said: "Interpretation is at the individual lender's discretion and some may view a payday loan as having a detrimental impact on some consumers' credit worthiness."
A spokeswoman for Wonga said: "We were aware that an old version of a customer email was temporarily in use again, following some software migration.
"We can confirm the email received by a MoneySavingExpert reader is not currently in use and we apologise for any confusion caused."
Earlier this week the trading watchdog announced it has launched formal investigations into several un-named payday lenders over the use of "aggressive debt collection" methods as part of a crackdown on the sector.
The Office of Fair Trading (OFT), which is carrying out a compliance review into the industry, is also writing to all 240 lenders in the market to outline its concerns about poor practices in the sector.
Earlier this week, Wonga apologised to campaigning MP Stella Creasy, after evidence was uncovered that an employee of the firm had been using an anonymous Twitter account to publicly attack her.
Writing on company website OpenWonga on Wednesday, editor Luke Manning said: "I want to make it absolutely clear that the actions of the individual responsible were inexcusable, and completely unacceptable.
"I also want to underline the fact that OpenWonga was unaware of, had nothing to do with, and disassociates itself entirely from the comments directed at Dr Creasy - which were made anonymously and without authorisation by another Wonga employee."
He said a disciplinary process was ongoing and added: "Dr Creasy has every right to campaign for the things she believes in, and although we may disagree with her view of Wonga, both Dr Creasy and her viewpoint deserve absolute respect.
"Wonga has made an immediate and unreserved apology to Dr Creasy, which we hope she will accept."
Reacting to the news when it broke, Creasy tweeted: "In this job, sometimes when people are out to get you, they really are out to get you."