LinkedIn is investigating reports that millions of users' passwords have been leaked online.
The reports, which originated from Norwegian IT site DN.no, say that over 6.4 million LinkedIn passwords have been posted on a Russian hacker forum.
The passwords, encrypted using SHA-1, are thought to have been leaked without any accompanying email addresses.
LinkedIn has not yet verified the breach, however.
After posting on its Twitter feed earlier today that is was "looking into reports of stolen passwords", it subsequently tweeted: "Our team continues to investigate, but at this time, we're still unable to confirm that any security breach has occurred. Stay tuned here. "
Nevertheless, users are being warned to change their passwords, and to be wary of phishers attempting to make hay from the alleged breach.
"It would seem sensible to suggest to all LinkedIn users that they change their passwords as soon as possible as a precautionary step. Of course, make sure that the password you use is unique (in other words, not used on any other websites), and hard to crack," said a post on security vendor Sophos' blog .