The media agency has reduced its April prediction by 1.8% after worse-than-expected economic conditions.
Unveiling its latest set of quarterly figures, Zenith also warned that UK media agencies could expect a further 1.4% decline in 2010, but predicted the market would recover the following year, with 2% growth.
This year, radio is expected to suffer the biggest decline, falling 15% to £368m. Newspapers will fare little better - ad spend across titles will decline 14.6%, says Zenith.
Jonathan Barnard, head of publications at Zenith, said that while the UK has not experienced the same level of paper closures as those in the US, he would "not be surprised to see some of the UK's regionals drop out in the second half of 2009".
But perhaps the biggest surprise in the report is that online advertising, which last year rose 19.1% year on year, is tipped to fall 1.8% in 2009.
Barnard pointed out digital should manage to increase its share of all media from 23.1% last year, to 25.4% in 2009, overtaking TV.
TV ad spend, where spots are being traded at their lowest levels for 20 years, is expected to fall 14% by the end of the year, despite relatively strong viewing figures.
Outdoor advertising managed to outperform the overall market. Zenith predicts that, while outdoor will fall 8% in 2009, it is expected to increase 3% in 2010 as media owners' investment, for example digital screens, begins to pay off.
The UK is underperforming compared with the overall global advertising market, which Zenith anticipates will fall 8.5% in total.
.jpg)
.jpg)