Google will today update its search algorithm so that it will penalise websites that do not optimise viewing on smartphones.
Originally announced in February, Google says the move is to adapt to the increasing number of people using mobile devices in order to search online, and the change will affect all searches done globally using Google on a smartphone.
Those sites that are not considered "mobile-friendly" by Google's new system will drop down search results.
A Google spokesman said: "When it comes to search on mobile devices, users should get the most relevant and timely results, no matter if the information lives on mobile-friendly web pages or apps. As more people use mobile devices to access the internet, our algorithms have to adapt to these usage patterns.
"Starting April 21, we will be expanding our use of mobile-friendliness as a ranking signal. This change will affect mobile searches in all languages worldwide and will have a significant impact in our search results.”
“Consequently, users will find it easier to get relevant, high quality search results that are optimized for their devices."
To help prepare websites for the change, Google has been offering resources that enable webmasters to test their mobile sites since the announcement was first made in February.
However, some major sites are yet to comply with the changes is about to introduce, meaning they are at risk of dropping down Google's search result pages, and see their web traffic fall.
Some market analysts are predicting that this year will be the first time that mobile searches surpass those on desktop, with more traffic coming from smartphone and tablets.
Somo's Noreen McCaffrey said: "This move further highlights the need to focus on what users experience when searching for your company on-the-go.
"While some brands think apps alone are enough to tick the 'mobile' box in terms of best mobile user experience, Google doesn't. As a result, these companies are about to see their mobile traffic declining."