The world’s leading web search firm, Google, on Monday announced that the unexpected bug that left as many as forty thousand Gmail users without access to their messages was caused by a storage software update.

Affected Gmail users were unable to access their messages because the software bug erased their email messages, folders, filters and labels. According to the firm, the bug affected 0.02 per cent of Gmail users.

Ben Treynor, vice president engineering at Google, said, "We released a storage software update that introduced the unexpected bug, which caused 0.02% of Gmail users to temporarily lose access to their e-mail.”

Speaking on the topic, Treynor added that they instantly halted the deployment of the new software when they discovered the glitch.

In addition, the firm admitted that the bug also affected a small fraction of business customers and that the firm would issue SLA credit.

The web search giant promised to set everything right at the earliest. But, it could take longer than usual as the glitch deleted affected customers' emails in multiple data centers. Google engineers are thus trying to restore accounts from backup tapes.