The annual value of pan-European online advertising is set to reach 16bn euros ($22bn; £10.8bn) by 2012, more than double that of 2006, says a study.
The report by research body Forrester said online adverts would leap to 18% of market share, up from 9% currently.
It said 52% of people were now regularly online, spending more time doing so than watching television.
The UK will continue to see the most online advertising in the next four years, ahead of Germany and France.
European internet users now spend 14.3 hours a week online, compared with 11.3 hours watching TV, and 4.4 hours reading newspapers or magazines, the research group said.
As a result of this increased internet usage, 36% of people who go online said they spent less time looking at the television as a result.
The report said search engines would continue to dominate online advertising spend, followed by display advertisements and e-mails.
"After five years of dipping their toes into the online marketing waters, firms have come to realise that the net is a valuable medium for client acquisition, retention and market expansion," said the study.