Zenith, owned by France's Publicis (>> Publicis Groupe), forecast growth of 3.6 percent in North American advertising spending in 2017 and an average of 3.4 percent a year 2019.

A slowdown in Britain would drag down spending growth in Western and Central Europe to 2 percent in 2017 from 3.9 percent last year, Zenith said, adding it expected average annual growth in the region of 2.6 percent to 2019.

The United States, the world's largest economy, would be the leading contributor of new ad dollars to the global market over the next three years and China would rank second, Zenith said.

Global advertising expenditure was expected to grow by 4 percent in 2017 reaching $558 billion, down from 4.8 percent in 2016, Zenith said. In June, it forecast growth of 4.2 percent in 2017.

Zenith said a slowdown in Britain, usually an outperformer in advertising spending in Western and Central Europe, would weigh. Regional spending growth would be 2 percent in 2017 and average 2.6 percent a year to 2019. It was 3.9 percent in 2016.

In June, Zenith forecast a 0.9 percent growth in British advertising spending in 2017, down from 9.6 percent in 2016.

Results from some advertisers have shown an upbeat picture in North America. Publicis reported a second quarter rise in underlying sales helped by North American revenues. Omnicom (>> Omnicom Group Inc.), the No. 1 U.S. advertising firm, reported better-than-expected quarterly results in July.

However, WPP (>> WPP Group), the world's largest advertising group, and Interpublic (>> Interpublic Group of Companies Inc) both reported weaker figures.

(Reporting by Noor Zainab Hussain in Bengaluru; Editing by Edmund Blair)