Websites which look bad on mobile phones will drop down Google's search rankings from today - cutting off a major traffic source.
Google is due to make a major update to its mobile search algorithm on Tuesday, in a move expected to cause ripples across the web.
Websites not deemed mobile-friendly will be dropped down the search rankings, dramatically reducing the number of visitors to a site.
Microsoft and the European Union are among the organizations likely to be negatively affected by the changes to Google's secret search formula.
People can check whether their website is classed as mobile-friendly by Google by entering the web address here.
Some search engine ranking experts have called the potential fallout from the changes "Mobilegeddon".
Itai Sadan, from website-building company Duda, told Business Insider: "I think the people who are at risk are those who don’t know about it.
"Come 21 April, a lot of small businesses are going to be really surprised that the number of visitors to their websites has dropped significantly. This is going to affect millions of sites on the web."
Companies spend significant amounts of money and effort to appear high up in Google rankings, with places on the first page of results highly prized.
Text that is too small to read on a mobile device, plus links that are too close together, are among the reasons some sites will fall down the mobile ranking list.
Google announced the changes in February, so companies have had just two months to prepare.
The firm said: "This change will affect mobile searches in all languages worldwide and will have a significant impact on our search results."