The global cost of ad blocking is expected to account for a $21bn loss in ad revenue this year, or a quarter of total global ad spend, a figure set to increase to $41.4bn in 2016, according to research from Adobe and PageFair.

The companies’ joint report, called ’The Cost of Ad Blocking’, found that in the US, ad blocking cost an estimated $5.8bn in lost revenue in 2014 and is projected to reach $10.7bn in 2015 and $20.3bn in 2016.

The findings revealed that the number of global ad block users had grown by 41% in just 12 months, with 198 million monthly active  users.

Ad blocking in the US grew 48% in the past year, to 45 million average monthly active users (MAUs) in Q2 2015, or around 16% of the US online population; meanwhile, across Europe, ad blocking grew 35% to 77 million MAUs.

According to the research, Google Chrome is still the main driver of ad block growth, due to the ease with which consumers can install ad block extensions, while mobile has yet to feel the brunt of ad blocking. But the report pointed out that “the release of iOS 9 in the fall of 2015 may be a game changer, as it will allow users to easily install ad blocking from the App Store”.

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

“PageFair is working with thousands of publishers to securely display user-friendly advertising and keep free websites in business. I hope this report will prompt more editors, website owners, and publishers to join with us to combat the problem.”

The full report is available here.