The Publicis Group agency’s new Advertising Expenditure Forecasts, published today, said advertisers would spend $99.3bn (£66.5bn) on mobile internet advertising in 2017, compared to £97.4bn on desktop.
In December, Zenith predicted that mobile would overtake desktop in 2018, but the mobile market grew quicker last year than anticipated.
Mobile almost doubled (up 95%) in 2015, while Zenith forecasts 46% growth this year, followed by 29% in both 2017 and 2018.
Mobile is already the primary means of accessing the internet, Zenith said in its latest Media Consumptions Forecasts 2016, and the media agency now believes the advertising market is catching up with the consumer.
As well as a surge in mobile, desktop advertising is shrinking, having peaked in 2014 at $98.9bn. Last year desktop shrank by 0.2% as advertisers switched their budgets to mobile, the report said.
Desktop is forecast to total $91.5bn in 2018, when it will account for 42% of all internet advertising, while mobile will account for 58%.
Growth in mobile is particularly strong in China and is forecast to account of 78% of its internet adspend by 2018, more than anywhere else in the world – and is set to overtake TV as China’s largest advertising medium. Next year China is forecast to overtake the US to become the world’s largest mobile ad market.
The UK is the second-most advanced mobile market in the world, behind China, and this year will be the only other market where mobile accounts for more than half of internet advertising. Mobile advertising will account for 28% of total UK adpend this year, rising to 39% in 2018.
But more generally, mobile advertising is still significantly behind television for all global market: £127.8bn compared to $192.2bn.
Zenith also reiterated its claim earlier this month that leaving the European Union could cost the UK £70m in terms of adspend growth each year. The UK's referendum on EU membership is on Thursday 23 June.
Jonathan Barnard, head of forecasting at Zenith, said: "China’s advertisers are leading the way in adapting to the rise of mobile technology.
"In China, mobile advertising is already the most important medium, and brand communications are mobile first. It’s only a matter of time before the rest of the world catches up."