One expert, commissioned by the BBC, estimates it could amount to “somewhere in the region of £1bn”.

The customers potentially affected had PPI on credit cards issued by Lloyds Banking Group, Barclays, MBNA and Capital One.

All claim to make every effort to pay a correct amount of compensation.

The extent of the shortfall is difficult to assess.

The shortfall in compensation arises because, although these banks all refunded the premiums on their mis-sold PPI policies plus interest as regulators require, they have been failing correctly to refund additional charges which were triggered by the premiums of the mis-sold PPI policies.

Some of those premiums put people over their borrowing limits, meaning they were then charged an additional fee.

This failure to include fees and charges in compensation calculations has resulted in dramatic reductions to the amounts some customers have received.

The Financial Conduct Authority (FCA) has told the BBC that it is already discussing the issue with the banks involved.

“If there are penalty fees or charges that arise from the mis-sale of the original PPI, then they should be refundable,” said Clive Adamson, the FCA’s director of supervision.

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One PPI claimant Mark Pascoe said he felt “ripped off”
‘Almost robbery’
In February, Mark Pascoe was paid £5,800 of PPI compensation by the large credit card company MBNA. But MBNA’s calculations did not include just over £600 in fees and charges Mr Pascoe incurred since taking out his card in 1997.

According to the claims management company advising Mr Pascoe, had those fees been correctly included in the calculations, his compensation payout would have more than doubled – to more than £13,000.

“It’s absolutely outrageous,” Mr Pascoe told the BBC. “That’s almost robbery, isn’t it? They should be fined big time.”

MBNA said they would not comment on a case still under review.

Martin Baker, managing director of Renaissance Easy Claim, the company advising Mr Pascoe, says his case is by no means atypical.

Mr Baker says he has identified a string of clients with PPI on their credit cards who suffered compensation shortfalls which run into many hundreds or thousands of pounds.

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“It was just another way by which the banks and lenders were trying to reduce their compensation bills,” says Mr Baker.