The update from the Public Groupe-owned media agency network comes after it had initially predicted a fall of 8.5% in July, but has since had to revise the figure due to weaker than expected first half results.
The media agency says it is expecting the rate of decline to improve in the second half of the year, with modest growth expected for 2010.
Ad spend in the UK is expected to fall by 13.1% this year to £10.7bn, higher than the 10.5% decline it had predicted in July.
However, this is predicted to ease next year with a 2.1% decline to £10.5bn.
In terms of media channels, ad expenditure on the internet is expected to grow 9% this year and reach a 14.9% share of global ad expenditure by 2011.
Meanwhile, newspaper and magazine ad spend is expected to continue to shrink, falling 25% and 28% respectively below their 2007 peaks, by 2011.
ZenithOptimedia says global ad expenditure should grow 0.5% next year, which is down from its previous forecast of 1.6%.
In North America ad spend is predicted to fall 12.6%, while in Central and Eastern Europe it will be down 20.9%. In Western Europe spend is expected to drop 11.2%, while in Africa and the Middle East, it will be down 11.4%.