The change, which comes into effect Tuesday, will see Google update the algorithm behind its search function to favour sites that are deemed "mobile friendly."
The change will not affect tablet or desktop searches, but mobile comprised nearly half of all traffic to Google between June and November 2014, according to marketing firm Clickz -- highlighting the importance of mobile for websites.
Given this, experts at marketing firm Somo said the change risks "mobilegeddon," as many websites look set to suffer in the search rankings. Even Google admitted it would have a "significant impact" on search results, in a blog post in February.
Google has released a tool – called the Mobile-Friendly Test -- where users can enter a website address and find out how the updated "Googlebot" views the page.
Research by marketing firm Somo showed that the websites of major companies, including Microsoft's Windows Phone and insurance firm Legal and General, would be affected negatively following the update.
Even the website of the U.K.'s security service, MI5, would be hit, as it too has a website that is not optimized for mobile.
Google's Mobile-Friendly Test says that the Windows Phone mobile site, for example, has "text too small to read" and "links too close together".
Ironically, the website of the European Union – which filed antitrust charges against Google and accused it of abusing "its dominant position" in the Internet search market – was also deemed by Google to be not mobile-friendly.
"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," Noreen McCaffrey, Somo's global head of marketing, wrote in a blog post last week.
Last year, Google issued another update to its search algorithm, called Panda. iProspect, a digital marketing agency, found the change affected between 10 and 12 percent of all websites, but Ben Wood, iProspect's global president, said the latest update could be worse.
"The general consensus is that this is a much bigger update," Wood told CNBC by phone.
"The truth is that large customer-facing organizations that haven't changed their website to be mobile friendly will find this to be wake-up call for them."