The passwords leaked to the Internet have been encrypted, but hackers are pulling together to crack them. Up to 300,000 of the stolen passwords have already been decoded.

LinkedIn has yet to confirm the claims, made by an anonymous Russian hacker, but tweeted that its tech team is "currently looking into reports of stolen passwords".

The site is used by 150 million professional business users worldwide. If the hackers have the associated e-mails linked to the stolen password, it could prove a valuable trove of high net worth data.

Security professional across the Web are recommending that LinkedIn users change their passwords as soon as possible as a precautionary measure.

The hack has also brought to light once again the importance of properly storing customer details. The database of passwords was encrypted using outdated SHA-1 encryption and were not 'salted' (where a random string of numbers is added to the encryption to increase the safety of the stored information).

Forensic experts at Manchester-based hosting company UKFast cracked 2000 of the passwords in just 10 minutes using only a standard computer's processing unit. With added power from a graphics card (GPU), this would be greatly speeded up.

Update: LinkedIn has confirmed the breach and updated its security protocols. Writing in the LinkedIn blog, Vincente Silveira states:

We can confirm that some of the passwords that were compromised correspond to LinkedIn accounts. We are continuing to investigate this situation and here is what we are pursuing as far as next steps for the compromised accounts:

Members that have accounts associated with the compromised passwords will notice that their LinkedIn account password is no longer valid.

These members will also receive an email from LinkedIn with instructions on how to reset their passwords. There will not be any links in this email. Once you follow this step and request password assistance, then you will receive an email from LinkedIn with a password reset link.

These affected members will receive a second email from our Customer Support team providing a bit more context on this situation and why they are being asked to change their passwords.

It is worth noting that the affected members who update their passwords and members whose passwords have not been compromised benefit from the enhanced security we just recently put in place, which includes hashing and salting of our current password databases.

We sincerely apologize for the inconvenience this has caused our members.