The customers are being asked to pay £5,000 each to fund the legal action.
People who took out shared application mortgages (SAMs) in the 1990s are planning court action against lenders - because they have been effectively trapped in their homes by the products.
Pressure group Struggle Against Financial Exploitation (SAFE) has sent a letter to its members, asking them to provide £5,000 each to fund a test case against the loans, the BBC reports.
SAMs were sold by Bank of Scotland and Barclays between 1996 and 1998. They work by offering borrowers around 25% of the value of their property interest free or at a very low rate. In return, the bank receives 75 percent of the difference in price between the time the home was bought and the time it is sold.
Effectively, this means that SAMs borrowers are left drastically out of pocket if they want to sell their home, due to the house price boom which occurred at the turn of the century and extended until the onset of the credit crunch last summer.
Speaking to the broadcaster Hilary Messer, director of solicitors Richard Wilson Pangbourne, said: "If the courts take the view, and we think they will, that the relationship between the homeowner and the bank is unfair that opens the door for the contracts to be revisited and for the court to substitute what they believe is a fairer outcome."
However, some SAFE members have objected to the letter, both for its hectoring tone and its financial demand. Responding, Elaine Williams at SAFE said: "We had to be upfront with people. I have had people say I have sold a ring and the cheques in the post.
"We have always worked to help SAM holders and now we have the best chance in history of doing this."
