A review of all the spam and fraudulent phishing e-mails blocked by Clearmymail in December found that 41% were targeting NatWest customers. Clearmymail says an average of 18 fake Natwest phishing e-mails were sent to its customers during the month.

Citi came second in the table, with 27% of fraudulent e-mails targeting customers of the US bank. An average of 12 fake Citibank e-mails were sent to Clearmymail customers during December.

HSBC came third in the table, with 11% of blocked spam featuring the bank's name. Clearmymail customers were sent an average of five fake HSBC e-mails during last month.

HSBC was followed by Abbey which featured in six per cent of blocked messages and Lloyds TSB, which was named in four per cent of messages.

Commenting on the study, Dan Field, MD of ClearMyMail, says: "The phishing e-mails used are very well constructed and often look exactly like a legitimate message from the bank."

Field says cyber-crooks are building up a portfolio of e-mail databases containing information that is profiled to fit a certain bank or building societies' "typical customer" in order to improve the success rate of attacks.

"Without any email protection UK online banking customers are at severe risk of being tricked into giving away their account details to criminal gangs," adds Field.

PayPal came sixth in the Clearmymail study, with just two per cent of spam purporrting to be from the eBay subsidiary. This is in contrast to an earlier 2006 US study by IT security firm Sophos which found that over 75% of all phishing e-mails caught in its spam-traps were targeting users of PayPal or eBay.