The company said initially that about 160,000 people were affected by the bug, but has now revised this figure downwards to 40,000.
The gaffe comes just weeks after a similar incident at Microsoft, in which hundreds of thousands of Hotmail users were blocked out of their accounts.
Ben Treynor, Google vice-president of engineering, wrote in a company blog: “When we discovered the problem, we immediately stopped the deployment of the new software and reverted to the old version.
“The good news is that email was never lost and we’ve restored access for many of those affected. Though it may take longer than we originally expected, we’re making good progress and things should be back to normal for everyone soon.”
Treynor says that even though Google keeps multiple copies of Gmail users’ data in different data centres, bugs can affect several copies of the data. It also holds back-up tapes, but restoring this data takes longer.
Google says it will post a detailed incident report outlining what happened, as well as action the company is taking to prevent it from occurring again.
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