It's been a bad 24 hours for Google's Gmail.

First there was Tuesday's outage that left both businesses and individuals with no access to emails for a number of hours. Now IT security firm Sophos is warning Gmail users about phishing attacks targeting them, and being spread via the Google Chat system.

Samples intercepted by SophosLabs revealed that Google Chat users were receiving unsolicited instant messages, urging them to "check out this video" by clicking on a link via the TinyURL service.

However, the link actually directed users to a website called ViddyHo, where users were asked to enter their Gmail usernames and passwords.

Sophos has warned that hackers could then use this information to break into accounts and steal information to commit identity theft.

"We're all used to receiving suspicious communications via email, but these attacks arrived via the instant chat system built into Gmail. As a result, more users may fall unwittingly into the trap", said Graham Cluley, senior technology consultant at Sophos.

"If you think you might have been duped, make sure you change your Gmail password immediately otherwise your entire address book and all your correspondence, including information that you may have archived about other online accounts, will quickly become rich pickings for the hackers".

Sophos research has also shown that 41% of computer users have the same password for a number of sites, and has therefore also urged users to change the password for these as well.

TinyURL has now blacklisted the site, meaning that the link will no longer work, but there are still plenty of URL shortening sites out there for the hackers to use.

Cluley added: "The message is simple. You should always be wary of clicking on unsolicited links whether received over email or IM, and be extremely careful whenever a website asks you to enter your username and password for another site".