Since the March edition of the forecast, which covers 12 major economies, advertiser sentiment has soured, according to Warc.
Suzy Young, data editor at Warc, said: "Global economic conditions remain unsettled and this is making marketers cautious, particularly in Western Europe, Japan and the US.
"As a consequence, marketing budgets for 2011 are likely to be smaller than initially hoped. The next three months will be crucial."
At constant 2005 prices total adspend in the 12 surveyed markets will actually decreases by 0.1% year on year.
China, due to grow 12.2%, will overtake Germany this year to occupy the number three slot a year earlier than Warc previously predicted.
The prediction for 2012 is for 5.7% growth.
Warc is due to release its consensus global adspend forecast, which collates third-party forecasts, later this month.
The last consensus forecast, in May, was 5.1% growth in 2011 and was raised from a prediction of 4.6% made in November.