About five million Google accounts and their passwords, mostly of Russia-based users, are suspected to have been leaked online.
At least 60% of the compromised accounts are said to be active.
The text file containing the database of all the accounts has been uploaded to social media platform Bitcoin Security Forum, with the user named tvskit first pointing out the appearance of the leak.
The details of names and passwords could provide access not only to Gmail but also to other Google services as well.
According to the Russia-based CNews, a spokesperson for the Russian office of Google Svetlana Anurova said: "experts now understand what happened in the case". She also urged users to "select strong passwords and be sure to use a two-step authentication".
The latest leak has surfaced a day after a similar disclosure emerged revealing at least six million accounts of Mail.ru, a Russian internet service provider, were reportedly stolen.
When contacted by IBTimes UK, Google said in a statement: "The security of our users is of paramount importance to us. We have no evidence that our systems have been compromised, but whenever we become aware that an account has been compromised, we take steps to help our users secure their accounts."
Following the allegations, the tech giant has released a blog-post which read: "We found that less than 2% of the username and password combinations might have worked, and our automated anti-hijacking systems would have blocked many of those login attempts. We've protected the affected accounts and have required those users to reset their passwords."
Story updated at 11:30 GMT on 12 September with Google's blog.