The research - conducted on the IAB's behalf by PricewaterhouseCoopers - estimates that total spend reached £3.5 billion ($5.2 billion) in 2009, up from £3.5 billion ($5.1 billion) in 2008.

Unsurprisingly, paid search was one of the biggest growth drivers, as marketers concentrated on highly measurable channels throughout the year. Overall, the IAB estimates paid search grew 9.5 percent year-over-year, accounting for over 60 percent of all online ad spend.

By contrast, overall display ad spend dipped 4.4 percent during the course of the year, representing around 20 percent of the overall market. Within that category, however, video witnessed striking growth, with advertisers spending 140 percent more on the channel in 2009 than they did in 2008. That suggests spending on more traditional display ad formats, such as banners and buttons, actually dipped by far more than 4.4 percent. The IAB also saw substantial strength in the affiliate marketing space, which rose by 38.2 percent, year-over-year, to reach £72.6 million ($110 million.)

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