Nationwide offers the UK's most secure online banking experience and Norwich & Peterborough Building Society the least, according to Which?
The consumer group used volunteers to carry out a series of tests on the Web sites of 12 banks and building societies to gauge their security.
Banks have been awarded a score out of 100 based on factors such as the ability to protect against keyloggers; whether users can stay logged in and browse to others sites; whether they need card readers; and if they are offered free security software.
Nationwide has come out on top, despite only managing a score of 69%. Natwest/RBS scored 63%, Barclays 62%, the Co-op 59%, HSBC 58%, Clydsdale 56% and First Direct and Lloyds TSB both 54%.
Norwich & Peterborough Building Society faired worst, with a score of just 35%, behind the Halifax on 38%, Santander 44% and Smile 49%.
Santander was the only bank to ask for a full password, which could leave accounts vulnerable to keyloggers, although the bank has since made changes to its online service. Halifax scored poorly for logout security.
Richard Lloyd, executive director, Which?, says: 'With so many of us doing our banking online these days, it's important that banks' security is up to scratch. We were alarmed to find significant flaws in the online security of some of the UK's biggest banks."