LinkedIn has launched an investigation into reports that its password database has been compromised with more than 6 million users' details posted online.

The business social network is examining claims by security analysts that millions of encrypted passwords have been published on a Russian hackers' website.

Graham Cluley, the cyberthreats expert, said the passwords were now likely to be in the hands of criminals. He advised the website's 160m worldwide users to immediately change their login details.

The security scare will cause fresh embarrassment for LinkedIn, which is also facing privacy concerns about its mobile calendar application.

LinkedIn has confirmed that some passwords have been compromised, and "apologised for the inconvenience this has caused our members".

Per Thorsheim, the security researcher who first raised the alarm about the apparent leak, said on Wednesday that the 6.5m encrypted passwords "will probably be a lot more users" because some will have the same login details.

Cluley, an analyst at Naked Security, said in a blogpost that users' emails addresses had not been published on the Russian hackers' website.

But he added: "It is reasonable to assume that such information may be in the hands of the criminals. As such, it would seem sensible to suggest to 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."

Those accused of being behind the breach have asked other hackers to help decrypt the protected passwords. If linked to a premium users' account, the hackers could gain access to credit card details used to pay for subscriber access to the site.

The file posted online reportedly contains 6,458,020 so-called "SHA1 unsalted password hashes", which would be straightforward for a skilled hacker to link to a user's details.

Earlier on Wednesday 6 June, LinkedIn was forced to fend off privacy concerns about its mobile calendar app. The company said its iPhone and iPad app can access detailed information about its users' daily activity, including location, meeting notes, and the email addresses of people a user meets with.