Google Chrome users on Windows-operated machines have been prohibited by Google from installing plug-ins not downloaded from the Chrome Web Store.

The plug-in block took effect on May 27, and will affect the stable and beta versions of Chrome on Windows. 

This action by Google, however, will most likely not affect too many users. The ones that will be affected the most are the users that like to use experimental add-ons to their Chrome browser. If you are one of these users, you will see that these plug-ins have already been disabled. 

Google's move to block plug-ins that were not downloaded from the Chrome Web Store is due to the increasing focus of the company on the user's security. Blocking plug-ins downloaded from other sources will help reduce the danger of users being tricked into downloading and installing harmful extensions. Google, however, added that disabled extensions may be re-enabled once the plug-in is able to gain hosting within the Chrome Web Store.

It will not affect other Chrome versions such as Windows Canary and developer builds, and Chrome on other operating systems such as Chrome OS, Mac and Linux. Google said that the company has no plans of extending the plug-in block to other platforms as most of the problems regarding malicious extensions are coming from Windows users.

The enforcement of this decision by Google comes as the company took down apps and extensions based on the Netscape Plugin Application Programming Interface, or NPAPI, from the Chrome Web Store.

Google's purging of NPAPI extensions from Chrome is part of the company's multi-year plan to increase the speed and security of the browser. NPAPI plug-ins potentially introduce instability and security problems to the browser, along with having a very complex code base.

Popular NPAPI plug-ins include Java, Microsoft Silverlight, Unity and even Google's own Google Earth.

Google announced this plan in September, and has since seen improvements in reducing the usage of NPAPI extensions.

"Over the last few quarters, we've been encouraged to see an overall 12.9% drop in per-user instantiations of NPAPI plug-ins and declining usage of the most popular NPAPI plug-ins," reported Google software engineer and plug-in retirement planner Justin Schuh at The Chromium Blog.

Google aims to have removed all NPAPI apps and extensions from the Chrome Web Store by September this year. However, existing installations of such apps and extensions will continue to be operational until Chrome fully removes NPAPI support.