Google is to punish mobile sites serving “intrusive” full-page interstitial ads by pushing them down mobile search rankings.

The firm has posted a blog warning publishers with pages showing interstitials risk providing a “poorer experience to users”, especially given the size of mobile screens.

From 10 January 2017, pages where content is not easily accessible to a user on the transition from the mobile search results “may not rank as highly”.

Examples of ad formats falling foul of Google’s new criteria include popups that cover the main content and standalone interstitial that the user has to dismiss before accessing the main content.

However, other forms of popup, such as for cookie usage, site log-in or age verification, will not be punished.

“Although the majority of pages now have text and content on the page that is readable without zooming, we’ve recently seen many examples where these pages show intrusive interstitials to users,” said Google product manager Doantam Phan.

“While the underlying content is present on the page and available to be indexed by Google, content may be visually obscured by an interstitial. This can frustrate users because they are unable to easily access the content that they were expecting when they tapped on the search result.”