Last month Group M forecast internet advertising would only hit the landmark 60% figure next year.

Publicis-owned Zenith estimates further growth of 3.8% in the UK internet advertising market next year, compared to the 1.8% growth for the UK market as a whole.

By 2020, Zenith expects internet advertising to account for 64% of total adspend in the UK.

The dominance of internet advertising in the UK far outstrips the global average. Zenith expects internet advertising to capture a 40% share of the global adspend market in 2018 and 44% in 2020.

Its value will rise from $203bn in 2017 to $225bn in 2020. The most advanced markets for internet advertising are Sweden and UK, followed by Australia, Canada, China, Denmark, Norway and Taiwan.

Zenith says its proprietary Touchpoints ROI Tracker tool, which is designed to compare internet ad spend to "internet brand experience", has found that ROI has now caught up with spend.

In 2016, internet advertising accounted for 34% of global ad budgets but produced 35% of brand experience, according to the tool.

Most of the growth in digital adspend is being captured by just five big platforms – Google and Facebook, plus the Chinese platforms Baidu, Alibaba and Tencent.

Between them the five platforms increased their share of global internet adspend from 61% to 72% between 2014 and 2016, and captured 83% of the growth over the same period.

In dollar terms, most of the growth in global adspend is coming from a few large markets.

Just two countries – the US and China – will contribute 47% of new ad dollars between 2017 and 2020, according to Zenith forecasts.

Meanwhile, big cities are driving global adspend by concentrating growth in productivity, innovation and trade.

Zenith forecasts that the top 10 global cities alone will contribute 12% of all adspend growth this year, and that the top 725 will contribute 60%.

Zenith global brand president Vittorio Bonori said: "We are seeing a battle played out in business, marketing and media between big players and small players.

"Growth is coming from big countries and big cities, and being captured by big platforms. Brands should focus on upstream strategy, data-informed UX planning and downstream automation".