Nationwide has been ranked as the safest online bank, ahead of Natwest in second and Barclays in third place.
Which? gave Nationwide an overall security score of 69 per cent, six per cent ahead of Natwest and seven points in front of Barclays.
Which? tested banks' security by recruiting 12 volunteers, who tested the banks' customer-facing security measures such as passwords and preventative measures for high risk tasks. They also used a keylogger to see how much information could be captured.
We were alarmed to find significant flaws in the online security of some of the UK's biggest banks.
Nationwide scored well in most areas, apart from in allowing users to visit other sites whilst banking online. This could lead to users downloading viruses during banking session, and is dangerous if they share computers with others.
The results showed that whilst no bank was so bad users should stop banking online altogether, none were able to protect from all fraud.
Some major banks were found low down the list as well, with Lloyds in eight, Santander 10th and Halifax 11th. The latter bank was described as having "surprisingly poor" security.
The smallest provider tested, Norwich & Peterborough Building Society, came in last with the weakest security.
"We were alarmed to find significant flaws in the online security of some of the UK's biggest banks. For example, Santander was the only bank to ask for a full password, which could leave accounts vulnerable to keyloggers, while Halifax scored poorly for logout security," said Which? executive director Richard Lloyd.
"With so many of us doing our banking online these days, it's important that banks' security is up to scratch."
Bank users aren't just at risk from lax provider behaviour. Earlier this month, 800 records with bank account details were left on an unencrypted USB stick in a London pub by a housing company contractor.
