Two-thirds of surfers are opening themselves up to fraud by reusing their online banking logins on other less secure sites.
Nearly three-quarters (73 per cent) of people recently surveyed admitted to using the same login details for their online banking across other websites and social networks.
Trusteer, the customer protection company for online businesses, said that by reusing these credentials, people put themselves at a serious risk of fraud.
Reusing passwords was the most common mistake made by those asked but 47 per cent also admitted to using exactly the same user ID and password to login to less secure sites.
Criminals have devised various ways to get login details from less secure sites such as online email clients and social networks and then test these credentials on financial sites to commit fraud.
Amit Klein, chief technical officer of Trusteer and head of the company's research organisation said: "Our findings were very surprising, and reveal that consumers are not aware, or are choosing to ignore, the security implications of reusing their banking credentials on multiple websites."
The company recommends that people have a least three sets of credentials to use across the internet.
The report comes in the same week that the Office of Fair Trading (OFT) began its scamnesty campaign, encouraging people to report any fraudulent emails they receive.