France-based ad tech company Criteo SA’s CRTO -4.61% shares plummeted Thursday after the company said a recent move by Apple Inc. to strengthen its ad tracker prevention is going to negatively affect Criteo’s revenue more than anticipated.

Shares were down 26% shortly before 2 p.m. in New York.

Criteo had been utilizing a workaround to Apple’s feature called Intelligent Tracking Prevention, which rolled out on the Safari internet browser on Sept. 19. The privacy-focused function, which is switched on by default, limits third-party access to cookie files that are stored on users’ devices. Marketers and advertising technology companies, including Criteo, use cookies to target and measure their web ads.

On Thursday, Criteo said iOS 11.2, the new version of Apple’s mobile operating system rolled out earlier this month, disables the solution it had been using to mitigate the effect of the tracker prevention function.

Criteo said if it cannot build an alternative solution, Apple’s tracking prevention could affect its 2018 revenue, minus traffic acquisition costs, by 22%. Criteo is particularly affected because it specializes in an advertising method called “retargeting, which involves serving ads to people who have already visited a website to remind them to return. The method relies heavily on tracking users as they browse from site to site.

In November, during Criteo’s third-quarter earnings, the company said Apple’s tracker prevention tool could have a 9% to 13% impact on its revenue in 2018. Criteo said Thursday it will provide formal revenue and profit guidance for 2018 in mid-February, when it reports its fourth-quarter earnings.

In an interview, Criteo CEO Eric Eichmann said Apple’s move could hurt publishers that generate money from selling ads and retailers looking to use data to drive shoppers back to their websites.

Mr. Eichmann said Apple has not introduced such strict anti-tracking measures within apps.

“They obviously are not against advertising for app developers and for the app environment,” he said. “In the browser, they don’t really care about it. They don’t get any benefit, they want to push you to the app environment.“

Apple did not immediately respond to a request for comment.

Criteo’s initial workaround involved using HTTP Strict Transport Security Protocol, usually used to secure web connections, that allowed it to create a user ID without cookies and match that with its own data.

Before doing so, Criteo would display a message to users informing them their browser no longer supported cross-site tracking and that clicking any link on the page would enable its tracker technology. Criteo also offered a link to disable its ad tracking.

Mr. Eichmann said this time around, Criteo is likely to be more “timid” in talking publicly about the technical aspects of its alternative solutions, for “competitive reasons” and because some of its technologies may have patents associated with them.

Apple’s Safari browser has a 3.3% share of the global desktop browser market and a 28.1% share of the mobile browser market, according to analytics website NetMarketShare.

Write to Lara O’Reilly at lara.o'reilly@wsj.com