Computer users are being urged to change passwords because of an internet security problem.
It is feared a bug known as "Heartbleed" has left sites, including those with the "secure padlock" icon, open to being hacked.
It is being blamed on a fault in the code which has been specifically written to protect financial transactions.
"What's happened with Heartbleed is that the technology that allows this security to be implemented on the web servers is actually vulnerable," said Conor Flynn from Information Security Assurance Services.
"There was a mistake by the programmers, and what this mistake allows a hacker to do is to actually decrypt or un-encode your secure information as it's going from your web browser to the server."