Businesses around the world are set to spend more advertising on the internet than on television for the first time in 2017, forecaster Zenith Optimedia has said.

Zenith said it expects internet advertising to grow at more than three times the rest of the industry in 2016, driven by demand for ads on social media, online video and paid search.

The forecaster had said in December it expected ad spending on the internet to overtake TV only in 2018.

"The global economy faces clear challenges," it said.

"But advertisers' confidence has remained largely unshaken, and our forecasts for global growth in 2016 have barely changed since we published our last forecasts in December."

Zenith Optimedia, owned by France's Publicis, trimmed its expectation for growth in global advertising this year to 4.6% from a forecast of 4.7% it gave in December.

That is still ahead of 3.9% growth in 2015.

The industry is expected to be boosted this year by advertising around the Rio Olympics, Euro 2016, and the US Presidential election.

However conditions in China, Russia and Brazil, along with uncertainty regarding Britain's membership of the European Union, are among the challenges to the global economy, it said.