The forecast decline for 2009 follows a drop in spend of around 1.8% last year, according to Zenith, and equates to the UK ad market's single biggest contraction since the Publicis Groupe agency began tracking spend 30 years ago.
UK internet display spend is set to record its first fall, of 1.1%, in 2009. The amount generated by display ads, such as web banners and skyscrapers, is expected to total £563m in 2009, down from £569m in 2008.
Similarly, spend on online classified ads, much of which has migrated from the UK press in the past five years, is also set to record its first fall, to £577m, from £607m in 2008.
The drops end five years of consecutive double-digit growth for display and classified ads. Meanwhile, revenue generated by the search sector, dominated by Google in the UK, is expected to increase 8% to £1.97bn.
Mauricio Leon, trading director at ZenithOptimedia, said these drops can be partly attributed to the depressed economy and partly to the internet landscape maturing.
He added: "Substantial growth was always going to have to stop at some point. There is still plenty more to come from online, with the next push coming from brand money as fast-moving consumer goods' advertisers recover."
Jonathan Barnard, head of publications at ZenithOptimedia, said the latest ad forecasts highlight drops of "nearly twice the magnitude" of those following 9/11 in 2001 and the previous biggest annual fall of 4.9% in 1991.
Total UK ad spend is set to return to growth, however, in 2010, with a 2.3% increase forecast.