Google, Microsoft, Amazon, and Taboola are paying the owner of Adblock Plus to unblock ads on their websites at a fee of "30% of the additional ad revenues" they would have made were ads unblocked, the Financial Times reports.

Adblock Plus is the most popular browser extension to block advertising and has been downloaded more than 300 million times. The free service says on its website that it blocks “annoying” banners, pop-ups, and video ads. Eyeo, the German company that owns Adblock Plus, says the add-on has more than 50 million monthly active users.

Ad blocking is an existential threat to internet services who rely on advertising for most of their revenues. As the Financial Times points out, German media groups including RTL and ProSiebenSat.1 are seeking damages from Eyeo.

To block all ads, Adblock users must go into the software settings and configure them to the strictest options possible.

Eyeo's business model operates around its "whitelist" service, which allows some ads to appear to Adblock Plus users. That whitelist consists of only "acceptable ads" that meet criteria such as being transparent about the fact they are advertising and not disrupting or distorting page content.

The whitelist is free for smaller sites, but a media-company source told the Financial Times that bigger internet companies and publishers were being asked to pay a fee "equivalent to 30% of the additional revenues it would make from being unblocked." The "acceptable ads" program has been running since 2011, and Business Insider has reported about it before, but this is the first time we have gotten an insight into how much companies are paying to be included in it.

The larger companies paying a fee to be whitelisted include Google, Microsoft, Amazon, and Taboola (a company that offers "sponsored content" links on sites including Business Insider, The Atlantic, TMZ, and MailOnline.)

Those deals could run the risk of irritating Adblock Plus users who may have been under the impression that the service blocked all online advertising but will notice ads appearing across some of the world's most popular websites. Those users can, however, choose to opt out of Adblock Plus' acceptable-ads program by unchecking an "Allow non-intrusive advertising" option on their browser extensions. They can also report ads they deem unacceptable.

Business Insider contacted Eyeo for comment about the Financial Times report.

Eyeo spokesman Ben Williams told us: "In general, I'm just a little surprised that our Acceptable Ads initiative is being treated as something new. To be as transparent as possible, we've been talking about it since it began in 2011. In addition, it's been widely reported; we wrote a manifesto about it and got others to join; we published blog posts explaining it in great detail; we've debated it; spoken about it on stages in Europe and America; and we put every applicant to the program completely out in the open on our forum."

He added: "It's also important to point out that we whitelist 90% of the entities in Acceptable Ads for free, that the same criteria are in place for payers and non-payers alike and that users can turn it off and block all ads any time they want. So in the absolutist world of ads-versus-ad-blockers, we're proud to offer a compromise that can encourage better ads and keep the free internet alive."

We asked Williams to clarify the amount companies like Google were paying to get their ads unblocked with Adblock Plus, but he responded: "We are not disclosing that information."