While almost everyone and their dog gets excited about the new iPhone 6s and other gadgets to be announced by Apple on 9 September, an upcoming change to its iOS software could be just as important. Starting with iOS 9, which will run on the new iPhones, Apple's Safari web browser will allow users to install ad-blocking software.

This software can stop adverts from appearing on websites as they browse the internet − an issue of much annoyance for users, but also the source of revenue for many websites. Those websites − particularly news organisations who have seen print revenues tumble and have struggled to make money by selling access to online content − will no doubt be worried.

Haven't we been here before?

Adblocking is not new. Many computer users have free browser plugins, like AdBlock, installed to prevent adverts from appearing on websites. Such adverts include banners, pop-up ads and video adverts that play before the start of YouTube videos. According to a 2014 report, over 140 million people use ad-blocking software, a figure up 70% year-on-year. The report claims 41% of 18-29 year olds block adverts from their browsers.

Adverts on mobiles − which are often full-screen pop-ups − can also be removed, but not as easily. iPhone users previously had to jailbreak their handsets, opening them up to the threat of malware, to block ads. Some third-party browsers on iOS and Android already block adverts, but these are used by a tiny minority compared to those who use Safari.

But the tide is turning, and even while this article was being written, Adblock Plus released an iOS web browser that removes all intrusive advertisements − a move the company claims has "beaten Apple to the punch".

With iOS 9 − which will be available to most iPhone users this month (September) and comes with the new iPhone 6s and 6s Plus when they go on sale − installing software to block adverts in Safari will be as easy as downloading a new keyboard from the App Store. Blocking adverts, especially those that include large images and auto-play videos, could help websites appear more quickly and lower consumers' monthly data usage, potentially reducing their mobile bill.

Blocking adverts also stops a website from knowing how long a visitor has been looking at each page, and how far down they scrolled before leaving.

Is this not a good thing?

For users fed up with full-screen adverts popping up, slowing down websites and automatically playing videos with sound, yes, the ability to block adverts in Safari is a good thing. But for advertisers and the owners of the websites that use them, the outlook is less positive. Many sites who give away content for free rely on the revenue earned through adverts to keep their businesses alive and their staff employed.

Blocking adverts on the desktop has already become widespread, and the fear now is that this will travel across mobile too - just as mobile becomes the more popular platform for consumers to read and consumer content online. Blocking adverts also affects those who provide them - chiefly, Google, which earns the vast majority of its revenue from selling online ads.

Just this week (5 September), users of the Adblock Plus Chrome plugin found they were unable to prevent adverts up to three minutes long from playing before YouTube videos. Only by switching Adblock off were they able to skip the advert after a few seconds.

If ad-blocking on mobiles becomes mainstream − and there is no reason to suggest it won't once millions of iOS users start installing blockers − then advertisers will have to think up new ways of getting through to potential customers. One alternative is Pagefair, a company selling a tool that can work around ad-blockers. Other companies offer 'whitelist' access, whereby only adverts that comply with a strict set of rules are allowed to appear on websites. These services will soon come to iOS 9, helping advertisers reclaim lost revenue and giving consumers better-designed and less intrusive adverts.

Who is the biggest winner here?

Apple is. The company has its own advert platform called iAds, but unlike Google, which shows adverts on websites, iAds only operates within applications. Now adverts can be blocked in Safari, expect to see more iAds appearing in apps − especially from companies that previously survived off web ads, who will now be looking to create apps to deliver their content alongside Apple's revenue-generating commercials.