The default browser for Apple's iOS operating system is Safari. When iOS 9 is released this fall, Safari will be getting its own downloadable extensions that will provide ad-blocking features, PC Mag reports. 

iOS 9 users will be able to download an extension that will block most ads on mobile sites. The updated Safari browser Content Blocking Extensions give developers "a fast and efficient way to block cookies, images, resources, pop-ups, and other content," Apple's website said.

Safari's engineer Ricky Mondello tweeted the news on Monday. Mondello says the extension will work throughout the browser. This means that as users browse the mobile web, they will not have to reset the extension each time they visit a new website.

Mobile advertisers are worried that an ad-blocking extension could take away from ad revenue. Nieman Lab calls the upcoming feature "worrisome" and said that mobile advertisers "already make tiny dollars on mobile."

The Content Blocking extension will also be available for OS X. Developers have the choice of using the Content Blocker section of the Extensions Builder or the setContentBlocker API.

More details on the ad-blocking extension are expected to be revealed at a developer session at Apple's WWDC on Friday afternoon.

AdBlock Plus, an extension that offers ad-blocking technology is worried about Apple announcing its own ad-blocking extension.

"Either this new API will improve Adblock Plus performance on Safari or it will force us to rely upon an inferior blocking format that would essentially kill adblocking on Safari," the company said.

"According to the announcements there are so called 'block lists,' which are JSON files, that can be registered by the extension," AdBlock said. "We are nervously awaiting how powerful [Apple's] block lists will be." But "if their block list format turns out to be useless, however, that could mean the end of adblocking on Safari."

On Monday, Apple started previewing iOS 9. Developers are now able to test out its features through a beta program. A public beta is expected in July.

Some new features coming to iOS 9 are: Intelligence (Apple's version of Google Now), split-screen multitasking on the iPad, transit directions in Apple Maps, an updated Notes App, and more support for Apple Pay.