The Scottish bank is the only one to cite data protection laws in failing to provide customer records dating more than six or seven years, according to a PPI claims firms.
The Herald reported last week that Beat The Banks, based in Dundee, claimed the policy was killing off hundreds of potential claims, and was not operated consistently.
It also produced evidence of the bank providing old records that it earlier denied having.
Paisley-based Payment Protection Partnership, the biggest firm in the Scottish market, also said client records more than six years old were said to have been destroyed in only 50% of its cases.
The revelations prompted a call by Dundee East MP Stewart Hosie for the matter to be investigated by the Treasury Select Committee, of which he is a member. Clydesdale Bank responded that its complaint handling policy was under review.
Now solicitor Gary Landa, former senior partner in Lanarkshire law firm Nelson Gibb & Landa, has said: "We have been representing clients in PPI claims for five years, and we have found Clydesdale Bank would do all it could to avoid settling claims.
"It would constantly claim records beyond six years have not been kept, or it had no record of the client whatsoever. It would claim signatures on mandates did not correspond with its records or a signature was not original when it was.
"The bank would make offers to clients who had multiple claims, offering on only two out of four claims on a 'take it or leave it' basis. The client was faced with a chance of an immediate few thousand pounds or taking it to the ombudsman and waiting wait up to two years."
Mr Landa said customers ended up "accepting offers therefore that were worth substantially less than there were rightfully due".
Colin Carr, of Glasgow law firm Carr Berman Crichton, which runs two PPI offshoots, said he had sent "hundreds of complaints" to the former Financial Services Authority about Clydesdale Bank's claims handling, and more recently to the new Financial Conduct Authority.
The FCA says organisations may have a policy of keeping records for six or seven years, but must still co-operate with any complaints or claims.
Mr Carr said the Clydesdale had in 2011 stopped accepting printed claims forms and would not deal directly with claims companies, effectively encouraging the customer to do it on their own.
He said that when the bank rejected claims, his firm achieved a success rate with the ombudsman of 50%. Mr Carr added: "We also believe that when a consumer proceeds on their own behalf, only approximately 10% who have cases rejected by the banks revert to the ombudsman."
Clydesdale Bank said: "We fully recognise our obligation to provide a fair and comprehensive complaints process, and continue to work with the FCA to ensure we have a robust complaints process that is in the best interests of our customers."