Microsoft is admitting that its in-house anti-virus product, Microsoft Security Essentials, had targeted Google's rival Chrome browser for blocking or even deletion, which the company insisted was unintended and caused by a software's bug during its routine definition update.

In a statement posted on its official site Friday, the software giant said the problem was identified and resolved within a day, although thousands of Chrome users were already affected.

"On Sept. 30, 2011, an incorrect detection for PWS: Win32/Zbot was identified and, as a result, Google Chrome was inadvertently blocked and in some cases removed from customers PCs," the Microsoft statement said.

"We have already fixed the issue. ... We released an updated signature (1.113.672.0) at 9:57 a.m. PDT," the company added.

To avoid further inconvenience, the company advised affected users to "go to the update tab and click the Update button, and then reinstall Google Chrome."

Microsoft also apologised for the problem it created, which according to The Australian had affected at least 3,000 Chrome users. The update was originally intended to neutralise the password-stealing Trojan Win32/Zbot.

"It allows limited backdoor access and control and may terminate certain security-related processes," Microsoft explained.

For its part, Google has announced over the weekend that a Chrome update would be deployed to fix the issue.

"We are releasing an update that will automatically repair Chrome for affected users over the course of the next 24 hours ... but if Chrome is working correctly for you, then there's no need to take any action," Google said on its blog site.