Customers of Lloyds, Halifax, Royal Bank of Scotland (RBS), NatWest, Ulster Bank, Barclaycard and HSBC received letters which were from either lawyers or debt collectors which were actually brands operating within these respective banking groups.
Banks said it was made clear in these letters the firms sending them were based in-house. But concerns have been raised that, as in the case of payday lender Wonga, some people may still have wrongly got the impression the debt had been escalated to an outside third party, putting them under pressure to pay up.
Banks said they have since stopped using the different brand names or are phasing them out. It also emerged some utility firms have also chased debts using the names of different brands based within their companies.
A key distinction between the letters sent on behalf of the banks and utility firms and those from Wonga is the letters involving banks and utility companies were from real organisations, while the Wonga letters were from firms which did not exist.
But Citizens Advice wants the Financial Conduct Authority (FCA) to consider whether, like in Wonga's case, people who have received such letters should get compensation, and Which? said the regulator should delve further into exactly how banks communicate with customers who owe them money.
Andrew Tyrie, chairman of the Treasury Committee, also voiced concerns, saying: "Customers should know who they are dealing with -it seems they may not have done. I will be writing to the banks for clarification."
The FCA said it is aware of reports about the incidents and it wants to hear from anyone who has further information about this type of practice. A furore erupted last week when it emerged Wonga had sent fake legal letters to customers.