Two per cent of cardholders in the UK - approximately 700,000 consumers - carry PIN numbers together with their payments cards, according to research by NOP World Financial on behalf of Halifax Card Services.
Furthermore, Halifax says a couple of those interviewed admitted to writing their PIN number on their actual card.
Commenting on the research, Ian Corfield, head of Halifax Card Services, says: "It's important that once you have your PIN number you either store it in a secure place or destroy it completely. If you're having trouble remembering your PIN you can easily change it to a number you can remember."
The survey also showed that 41% of respondents carried enough information in their wallets to allow their identity to be stolen. According to the research two out of five consumers carry some form of identification in the same place as their payment cards - usually in the form of a drivers licence.
Corfield adds: "Our research showed that a worrying number of us carried more than enough information in our wallets to allow would-be thieves to steal our identity. Fraudsters only need a few pieces of information and carrying it all in one place can therefore be very risky."
The bank recommends that consumers keep forms of identification - such as drivers licences - in a separate place to their payments cards and to take receipts home, check them against statements and then destroy them.