Websites with mobile pop-up ads, watch out. Google is cracking down on the "intrusive" advertising by knocking down those pages in its search results, the company said Tuesday.

Any website that shows a pop-up ad that covers the site's main content — whether right when the user gets to that page or while the user is on the page — will be ranked lower in Google's search results. That applies to pop-ups users have to dismiss to continue onto a site, and advertisements that cover the top half of the page.

Google's search engine drives millions of users a day to pages across the web, making it a powerful tool to attract visitors. Many websites rely on this traffic combined with display advertising — sometimes in the form of pop-ups or interstitials — to remain in business.

Now, Google is pushing websites to choose between a high search ranking and obtrusive but lucrative ads.

Google's examples of ads that will be penalized in mobile search results. Image: screenshot/google

Pop-ups that are part of a legal obligation — to verify a user's age or notify about cookie use — won't be penalized, nor will small banner ads.

The changes in search result ranking will take effect Jan. 10, 2017.

The new rules are applied specifically to mobile, where the ads are more problematic, Google says, because of smaller screens.

"Pages that show intrusive interstitials provide a poorer experience to users than other pages where content is immediately accessible. This can be problematic on mobile devices where screens are often smaller," Google wrote in a blog post announcing the policy. "To improve the mobile search experience, after January 10, 2017, pages where content is not easily accessible to a user on the transition from the mobile search results may not rank as highly."

Many publishers use pop-up ads that require the user to answer a question, skip ahead to the main page or scroll past a half-screen ad on both desktop and mobile.

Even though the factor is only one among hundreds that determine search result ranking, the importance of Google results to nearly every site could be enough to encourage publishers to change the common practice.

As social takes up more of people's time on their smartphones, Google is trying to make mobile better. Google's "Accelerated Mobile Pages Project" tried that first, by paring down web pages to make them simpler and faster, similarly to Facebook's Instant Articles.

Google is also eliminating a mobile-friendly label it introduced two years ago from its search results, now that 85 percent of pages meet the criteria to be considered mobile-friendly.