Britain's financial regulator is exploring the possibility interest rate products may have been mis-sold by banks, including Barclays.
The Financial Services Authority also said on Sunday that Barclays had apologised and was to write to customers affected.
The investigation came after the Telegraph newspaper provided the FSA with new information, leading the FSA to say it would look into how the products were sold.
We have looked at the issue before. We asked the firms involved to review their systems and correct any problems, the FSA said.
We will look into this new information from the Telegraph, and consider appropriateness of the product and how it was sold. If we find widespread evidence of breaches or mis-selling we will take action.
Barclays, which was named by the Sunday Telegraph in a report that said the bank had sold swap products to small businesses, was not available for comment.
The new FSA probe followed Britain's move in February to close a bank tax loophole which was disclosed to the government by an unnamed bank, said to be Barclays.
The watchdog said it was unacceptable for any firm to try and prevent customers speaking to the FSA, an activity to which it said the Telegraph had drawn its attention.
Hector Sants, the head of the FSA, said on Friday he had resigned and would leave in June.
(Reporting by Sarah Young; Editing by Dan Lalor)