The market research company has forecast that 20.9% of UK internet users would use the software in 2017, a substantial drop from last year, when it expected the rate to hit 27% by the end of this year.
eMarketer now forecasts that 22.2% of UK internet users, or 12.3 million people, will use ad-blockers by the end of 2018. It said the downward revision was because ad-blocking on mobile had not grown as quickly as expected.
Of the 11.4 million people using them this year, only 33.8% are doing so on mobile, compared to 89% who do so on desktop and laptop. Next year, the gap is expected to narrow, but only slightly, to 37.3% vs. 87.4%.
But Bill Fisher, senior analyst at eMarketer, warned that mobile ad-blocking could still prove to be a "ticking time bomb" for brands.
He said: "Our research has pointed toward the threat of mobile as a potentially huge disruptor. In Asia-Pacific, for example, mobile ad-blocking is a much bigger phenomenon than in the West.
"There are a number of variables at play here, but key amongst them is the fact that many mobile browsers in the AP region have ad-blocking extensions switched on as standard.
"Google’s plan to have an ad-blocking feature switched on as standard in its Chrome browser (desktop and mobile versions) will likely have a big impact on that mobile trend in the UK."
Google announced last week that from next year, Chrome would automatically block ads that were not compliant with the Better Ads Standard revealed in March by industry consortium the Coalition for Better Ads.
Another factor, added Fisher, was the high proportion of mobile time that UK users spent in apps. "The next big mobile disruptor, then, would be the emergence of easy-to-use and widescale in-app ad-blocking technology," he said.
"When one takes the demographic consideration into account, too – younger people are more likely to block ads, plus they’re more likely on mobile devices (in-app) – then the potential for mobile as a major disruptor becomes quite obvious."
eMarketer's data echoes research from the IAB and YouGov in February that found usage levels were stabilising, after signficant growth in recent years. But industry figures expressed scepticism when Campaign asked whether ad-blocking was now a declining problem.