Zenith, owned by France's Publicis (>> Publicis Groupe), had forecast a 2018 growth rate of 4.2 percent in September. It had also estimated 4 percent growth in 2017, down from 4.8 percent in 2016.

"We expect advertising expenditure to grow more slowly than the global economy as a whole out to 2020," Zenith said.

However, a boost of 4.1 percent would still be welcome news for global advertising giants such as WPP (>> WPP Group), which cut its sales expectations for the third time this year in October, with weak client spending and technological disruption putting it on course for its lowest growth since the financial crisis.

This bleak outlook from the world's largest advertising company followed similar forecasts from peers Publicis and Interpublic (>> Interpublic Group of Companies Inc).

Zenith said that growth in North American advertising spending is being lifted by Canada's buoyant economy and is outpacing growth in Western and Central Europe.

Advertising expenditure was forecast to grow by 3.4 percent in North America in 2018, down from the 3.6 percent growth it forecast for 2017.

Zenith also lowered its forecast for Western and Central Europe to 1.7 percent this year from 3.9 percent last year, hit by a slowdown in Britain.

Political and economic uncertainty over Brexit have contributed to a sharp slowdown in British advertising spending, which Zenith expects to grow by only 0.7 percent this year, down from 9.6 percent in 2016.

Zenith reiterated that the United States, the world's largest economy, would be the leading contributor of $72 billion in advertising spending to the global market between 2017-2020, with China ranked second.

Separately, WPP's GroupM said it expects global advertising spending growth of 3.1 percent this year and 4.3 percent in 2018, helped in part by events such as the football World Cup. (http://prn.to/2BEWFyi)

(Reporting by Noor Zainab Hussain in Bengaluru; Editing by David Goodman)

By Noor Zainab Hussain