By Paul Lewis
BBC Radio 4's Money Box
Increasing number of people are falling behind with their mortgage payments
Four mortgage lenders are being investigated for charging excessive fees to customers in arrears.
The Financial Services Authority (FSA) will not reveal their names until it decides to take enforcement action.
But the consumer body Which? wants the FSA to fine more firms to make them treat customers in arrears fairly.
The Council of Mortgage Lenders has told its members that charges should not exceed the cost of dealing with customers.
Jon Pain the FSA's managing director of Retail Markets revealed that the firms were being investigated but told MPs on the Treasury Select Committee that he would not reveal their names: "until the full enforcement process is complete it would be rather unjust to say they are guilty before they are proven guilty".
Dominic Lindley of Which? told Money Box on BBC Radio 4 the firms should be named: "The FSA should realise that a quiet-word-in-your-ear kind of regulation isn't working and lenders will only treat customers fairly when they see very high fines."
The Which? research found that almost all lenders charged fees for various services to customers in arrears.
Barclays charged a monthly fee of £40, Lloyds charged £31 for a second arrears letter, and Nationwide £95 for a visit from an arrears adviser.
"There's no evidence that these [charges] are a reasonable reflection of the cost and they can be charged regardless of whether you have entered into an agreement to pay off those arrears," Mr Lindley said.
Rules set down by the FSA say that mortgage lenders should ensure "arrears charges reflect a reasonable assessment of the cost of the additional work".
Sue Anderson of the Council of Mortgage Lenders told the programme: "Absolutely, they shouldn't be a profit stream. But we shouldn't assume that a certain level of charge is a profit stream.
Have you fallen behind with your mortgage?
What could be done to help people in arrears?
"They have to employ staff, set up computer systems and pay overheads so it is wrong to assume they are unfair."
But Treasury Minister Lord Myners told the Treasury Select Committee that "there is a risk that charges could be excessive and the FSA should give that very serious attention".
And Mr Lindley of Which? wants MPs on the Committee to: "recommend rules so that if a consumer has entered into an agreement to pay off arrears then all charges are suspended."
Which? also wants: "A 90 day charge-free window so customers can negotiate an agreement using an independent debt counsellor such as someone from CAB."
Sue Anderson from the Council of Mortgage Lenders said that customers with a complaint they could not resolve should "look to the Financial Ombudsman Service".
BBC Radio 4's Money Box was broadcast on Saturday, 11 July at 1204 BST