The bank was commenting on its unique practice of rejecting payment protection insurance (PPI) mis-selling claims on the grounds that it has destroyed all customer records more than six years old.
The Financial Conduct Authority is understood to be continuing to examine Clydesdale's PPI claims handling, while the bank said it was "well advanced with our review" of its policy.
Leading Scottish PPI claims companies revealed four months ago that Clydesdale was the only bank to invoke a 'data protection principle' in claiming to destroy all customer records after six or seven years, effectively killing off a large proportion of PPI claims.
They said in some cases the bank had managed to produce old records, and accused the Clydesdale of deliberately withholding information to avoid paying out tens of millions of pounds in PPI mis-selling compensation.
The firms said they had made repeated complaints to the financial regulator, prompting treasury committee member Stewart Hosie MP to promise an inquiry. Now Beat the Banks, a Dundee-based company run by former Clydesdale bankers, has challenged the bank to deny that it keeps microfiche records of customer transactions for 21 years.
Mike Begg, director of Beat the Banks, said he had reliable testimony on the issue, and had referred all his fresh evidence to the Financial Conduct Authority, which had indicated its proactive interest.
A Clydesdale Bank spokesman said: "We hold historic records that may go back 21 years, but not in all cases. We have paid out on a number of these older cases but accept that the approach has been inconsistent."
He went on: "We continue to work with the FCA to ensure we have a robust complaints process which is in the best interests of our customers."
Mr Begg said that following The Herald's reports he had a meeting with the bank early last November. "They promised no end of stuff they were going to do to resolve this - and it's worse than it has ever been before."
Two other firms that criticised the Clydesdale's procedures last October, Paisley-based Payment Protection Partnership, one of the biggest claims firms in the UK, and Glasgow law firm Carr Berman Crichton, which runs two PPI offshoots, both said there had been "no change" in the bank's procedures.
The law firm's Colin Carr has sent "hundreds of complaints" to the regulator about Clydesdale Bank's claims handling. Clydesdale's normal response to a request for data more than six or seven years old is that it cannot be produced "due to the passage of time", and that the bank destroys all records in accordance with a data protection protocol.
Mr Begg said RBS also destroyed paper customer documents, but was nevertheless able to produce loan and mortgage account records going back in some cases to 1995, which showed whether they included PPI.
"It is not that the Clydesdale don't have it," he said. "We did get one file recently for someone with personal information going back to 2002 - it was almost as if someone did not know what they were doing and gave us everything by mistake."
Mr Begg added: "If they are able to give me a list of accounts, but can't tell me whether they had PPI, that is beyond comprehension. The net result is that the poor punter is losing out."
He said older claims were potentially more expensive to a bank - a £2000 loan premium paid in 2000 would trigger a full refund plus £600 interest, plus 8% penalty interest a year for the past 14 years - a total liability for the bank of almost £5000.