Based on data from 26 markets, the trade body's latest AdEx Benchmark report suggested the internet ad sector was worth €20.9bn in 2011, equivalent to a 20% share of adspend across all media.
According to the organisation, the web thus vastly outperformed traditional media in 2011, as the latter channel witnessed a modest lift of 0.8%.
"Advertisers increasingly recognise online as a branding medium," Daniel Knapp, the report's author and head of advertising research at IHS Screen Digest said. "The explosion of 'big data' has delivered enhanced targeting capabilities, improving monetisation of publishers' inventory."
In all, Europe's five largest online ad markets - the UK, Germany, France, Italy and the Netherlands - contributed 67.9% of ad revenues in 2011, down from 69.2% the previous year.
Instead, growth was mainly driven by central and eastern Europe, which accounted for the seven largest year-on-year increases in national expenditure for 2011.
Russia led this group, up 55.5% to €1.12bn, meaning it is now Europe's sixth-largest internet advertising market. Serbia registered the second-highest growth rate, after a 46% increase.
This regional adspend trend reflected the general economic situation, as mature European markets struggle while central and eastern European nations record consistently higher GDP growth.
But central and eastern Europe still collectively accounted for a small minority of overall adspend, taking a share of 11.8% in 2011, compared to 10.1% in 2010, the AdEx report stated.
Knapp added: "There is a long term trend for advertisers to shift ad budgets from mature to emerging markets, which is fuelling their online economy. An expanding broadband infrastructure adds to the attractiveness of those markets."
Elsewhere, the analysis revealed that paid search remained the most lucrative ad format, taking a 46.5% market share in 2011. Display was next on 33.6% and directories yielded 19.2%, largely flat compared with 2010.
For the future, the report earmarked mobile for rapid growth. Mobile display adspend rose by an average of 45% year on year in the nine markets now measuring this channel, across which it claimed a 3% share.
Online video was revealed to be gaining similar traction across the continent, delivering 9.8% of display ads in Sweden and 9.7% in the UK. Eastern Europe also saw a strong performance here, with the format taking a 5.8% share in Slovakia and Poland.
Data sourced from Warc