Internet bank egg has denied cancelling the credit cards of customers who clear their balance every month because it does not make any money out of them.
The bank said it has sent letters to 160,000 people with a "higher than acceptable risk profile" informing them that their credit cards will stop working in 35 days.
Those with a higher risk would typically mean people who spend over their credit limit or fail to make minimum repayments.
But a number of people who have received a warning letter have insisted that they had excellent credit ratings.
Some speculated that egg wanted to get rid of them because it made no money from their good financial management - something the bank insisted was "absolutely not the case".
Labour MP Nigel Griffiths, a former Consumer Affairs Minister, called on the Financial Services Authority (FSA) to investigate the bank's move.
The bank began sending out the warning letters on Thursday to 7% of its two million credit card holders. It is not asking for immediate repayment of balances or making any changes to current terms and conditions.
A spokesman insisted the move was not linked to the credit crunch but followed a "risk review" of its customers which was carried out after investment bank Citigroup bought egg last year.
He said egg informed the FSA before carrying out its plans and would be happy to meet Mr Griffiths to discuss his concerns, the spokesman added.
Angela Knight, chief executive of the British Bankers Association, said egg's action was "a sensible way of looking after a business" and she had no reason to think it was prompted by the global credit crunch.
