The disclosure came after The Herald uncovered evidence from reputable Scottish claims companies that Clydesdale is the only major bank which says it is unable to provide customer account records dating back more than six or seven years.
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The bank has headed off large numbers of potential claims by saying it has destroyed the records of PPI-linked accounts closed before 2006 or 2007, citing data protection laws.
The claims firms say these obstacles show why many people are prepared to pay them a fee of 25% to pursue any compensation they may be due for mis-sold PPI.
But the firms also say most banks co-operate with requests for disclosure, suggesting that customers considering making a claim could do it for themselves.
The same applies to customers of the Clydesdale, given that the claims firms admit that when the bank will not supply information, claims tend to be abandoned anyway.
If the customer has any paperwork, he could be at an advantage in a complaint to the ombudsman, which upholds 70% of complaints.
Stewart Todd at the Financial Conduct Authority said: "Whilst we acknowledge that firms have their own data retention policies, this should have no impact on how they address a customer's complaint that stretches back beyond that point.
"If a consumer feels a complaint has not been handled properly they should then complain to the Financial Ombudsman Service."
The bank would then be expected to provide some information or documentation, and if it had none, it would find it difficult to argue against any documents provided by the customer.
Rory Stoves at the FOS said: "With PPI what we will try to do is build a picture based on the other evidence that has been provided. If someone has no documentation, it is going to be very difficult for either party to prove their side. Then we have to decide what is most likely."
The bank has eight weeks to respond, though the FOS may take a year due to its huge caseload. The average pay-out is £2,700.
The advice from consumer groups and the ombudsman is to dig out loan and credit card agreements and scan them for any mention of payment protection insurance - or a ticked box that may have gone unnoticed at the time. Bank or card statements may also show PPI premiums.
If you can't find all the paperwork but suspect you were sold PPI in the past, write to the bank or other lenders and request a list of your historic accounts and products.
Template letters and questionnaires are available from Which? (which.co.uk) or the ombudsman (financial-ombudsman.co.uk). There is no time limit for making a claim, and banks are obliged to keep a record for at least six years after the end of any loan agreement.
Rory Stoves at the FOS said: "We don't expect them to hold it indefinitely, if its within seven or eight years we would expect it but any longer than that and we wouldn't necessarily expect it."
He said a bank might not hold onto records where a customer had left and their account had been closed.
But according to the claims firms, RBS, Lloyds/Bank of Scotland and Nationwide can and do provide details going back to 1998 or earlier, in response to the initial correctly-filed request.
Proof of having been sold PPI does not prove it was mis-sold. But if it was added on to a transaction done in a branch, and/or you have no recollection of being told anything about PPI, the likelihood is you weren't told. If you were self-employed at the time, or had similar insurance in your workplace, it was a blatant mis-sale.
One retired Dundee woman, who does not want to be identified, has in recent weeks received almost £15,000 in reclaimed PPI payments after going to local company Beat the Banks. She said: "I knew nothing about PPI until lately, and in both the jobs I was working in at the time I got paid when I was off, so I didn't need it."
The bank was the Clydesdale, and it has disclosed records of all her 11 accounts dating back to 1997, despite the bank claiming in response to thousands of other requests for information that it has "destroyed the paperwork" for accounts before 2006. In some cases it has denied having any documents, then produced them.
Both Beat the Banks and Payment Protection Partnership, Scotland's biggest PPI claims firm which has reclaimed almost £100m from banks, say the Clydesdale's inconsistency shows that it is withholding records, which the bank denies.
Now the Treasury Select Committee is set to look into the issues after committee member Stewart Hosie, MP for Dundee East, said the allegations were "deeply troubling", as was the apparent lack of policing by either the FCA or the Information Commissioner's Office.
Clydesdale Bank has said it fully recognises its obligation to provide a "fair and comprehensive PPI complaint handling process which is not constrained by time period", and its process including information disclosure was under review.
It says it is "committed to delivering the best outcome for customers".