The firm has raised its growth forecast to 3.5% from 2.2%. The developed markets of North America and Western Europe are expected to grow only marginally, but developing economies should see much more significant year-on-year improvements.

In its last forecast Zenith predicted that the North American market would shrink by 1.5% this year but the firm has since upgraded this to growth of 1.3% as consumer confidence and spending in the region have “recovered quite strongly”.

In Western Europe the rate of growth has been upgraded from 0.4% to 2.2% “after advertisers demonstrated willingness to spend in the first half of the year”. The only country in the region not expected to see growth is Spain, which Zenith said is one of the most vulnerable Euro markets and has been downgraded from 0.4% growth to a decline of 1.7%.

The market in Japan, which Zenith said is “almost completely decoupled” from the rest of Asia Pacific, is expected to shrink 0.7% while the remainder of the Asia Pacific region is predicted to see 10.6% growth thanks to double digit increases in China, India, Indonesia, Malaysia, the Philippines, Thailand and Vietnam.

Zenith forecasts that global ad spend will reach $467.8bn in 2011 and $492.6bn in 2012.