Legal judgments have ruled that customers can claim 100% of the PPI cost back because commission levels were unfair.
But claimants wishing to recoup this money have to make a Data Subject Access Request to find out how much-undisclosed commission was levied as a first step towards bringing a claim.
Requests are often taking months to answer as banks are accused of deliberately sitting on a huge backlog of DSARs.
A letter to banks by MPs on the All Party Parliamentary Group on Fair Business Banking said such delays could be in breach of laws surrounding the DSARs.
It said: “There are real concerns that some banks may be deliberately delaying their responses to frustrate and stifle claims.”
It is estimated that such claims could cost banks £18billion in compensation, on top of roughly £40billion paid out to date.
William Wragg MP, co-chair of the APPG on Fair Business Banking, said: “If it turns out that banks are acting as a block to the proper legal process it would be a concern to us all.
“Whether the claim relates to PPI, swap mis-selling or any other matter, the legal process must not be stifled or delayed in any way.”
The fresh wave of PPI claims comes after it emerged that for more than a decade banks and credit card companies charged their customers commissions of up to 95%.
Law firm Harcus Parker last month launched a group legal claim that aims to help millions of PPI customers.
More than 350,000 people have already signed up.
Damon Parker, senior partner, said: “Banks are playing fast and loose with the law. In many cases they either require or request that customers make DSARs before launching their claims.
“Then they just sit on the requests, for months, even years. It is an outrageous abuse of the system and individuals’ rights.”
