The agency has turned far more bearish since its last forecast in December, blaming a rapid fall-off in world trade and saying the first quarter of this year was "at least as tough" as the fourth quarter of 2008.
It had predicted that global spend would fall 0.2% and UK spend would rise 1.5%.
Now it expects 2009 to see a severe downturn before 2010 brings recovery with global spend up 1.5% and UK spend also back to growth.
The UK will not be the only major market to suffer this year; the US is in line for an 8.3% drop, Germany a 5.5% drop and Japan a 5% drop.
All forms of media will see a reduction in global spend except the internet, which will grow 8.6% this year to $54bn.
In the US, the biggest internet market, ZenithOptimedia predicts search and classified will grow by 9% and 1.8% respectively but display will shrink 1.8%.
The report author Jonathan Barnard warns: "A number of internet companies that have based their business model on advertising may find their model unsustainable now that credit is drying up."
The internet's share of global spend will rise from 10.4% in 2008 to 12.1% in 2009. Television is the only other medium that will also grow in terms of share, from 38.1% to 38.6%, though actual spend will drop 5.5% to $173bn.
Newspapers' share will drop from 25.3% to 23.8% as spend falls from $122bn to $107bn.
For the first time, advertisers will spend more on the internet than on magazines, which will see spend drop from $55bn to $49bn.
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