The use of software that blocks online ads is expected to cost websites some $21.8 billion globally in 2015, a study showed Monday.

The study, by software group Adobe and Ireland-based consultancy PageFair, found that the number of Internet users employing ad-blocking software has jumped 41 percent in the past 12 months to 198 million.

The report said that while consumers have warmed to the idea of blocking online ads, they may not realize that the practice could hurt websites which rely on ad revenue.

Those losses are expected to grow to more than $41 billion in 2016, the study said.

"It is tragic that ad block users are inadvertently inflicting multi-billion dollar losses on the very websites they most enjoy," said PageFair chief executive Sean Blanchfield.

"With ad blocking going mobile, there's an eminent threat that the business model that has supported the open Web for two decades is going to collapse."

Consumers are able to install extensions on some Web browsers such as Google Chrome which block most ads. A similar tool is expected to become available on Apple devices with the release of the new iOS operating system later this year.

The report said that because of this growth, "ad blocking now poses an existential threat for the future of free content on the Internet."

In the US market alone, blocked ads resulted in $5.8 billion in losses in 2014 and are estimated to cost $10.7 billion this year, PageFair and Adobe found.

Campbell Foster, Adobe's director of product marketing, said he hopes the report sheds light on the online ecosystem

"Consumers, for the most part, accept the tradeoff that comes with 'free' -- I'll give you information about me in exchange for your TV show, film, news article, or service -- but draw the line at advertising that's intrusive, annoying, irrelevant or downright creepy," he said in a statement.

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ADOBE SYSTEMS