Documents seen by CNET UK suggest thousands of usernames and passwords for Hotmail, Google and Yahoo accounts have been illegally posted to the Internet.
Login credentials for accounts ending with yahoo.com, hotmail.com, gmail.com, msn.com, live.com and hotmail.fr were seen. Users of these services are strongly encouraged to immediately change their passwords.
Usernames and passwords for Google's Gmail service could also provide hackers with access to users' YouTube, Blogger, Google Docs and Google Talk accounts, as these services are all owned by Google and often work under a single login ID.
We contacted Google, which acknowledged the leaked details and blames phishing attacks rather than insecurities within Google's system.
"We recently became aware of an industry-wide phishing scheme through which hackers gained user credentials for Web-based mail accounts including Gmail accounts," a Google spokesperson told CNET UK.
"As soon as we learned of the attack, we forced password resets on the affected accounts. We will continue to force password resets on additional accounts when we become aware of them."
We also contacted Yahoo, a spokesperson for whom confirmed, "We are aware and are investigating."
Reports of leaked Hotmail account details first appeared on Neowin. Microsoft later confirmed the news, and announced that "as a result of our investigation we are taking measures to block access to all of the accounts that were exposed and have resources in place to help those users reclaim their accounts."
Once again, if you have email accounts with Google, Microsoft or Yahoo, you are strongly advised to change your password immediately.
We will update this story as and when we hear further details.