The report found that internet expenditure grew to £1.75 billion, making up 23.5 per cent of the total.
The vast majority of that figure comes from search advertising.
TV took 21.9 per cent of the market, with press and DM on 18.5 per cent 11.5 per cent respectively.
Overall, ad spending fell by 3.5% year-on-year to £17.5bn.
Guy Phillipson, the chief executive of the IAB, told Reuters: "This is a significant milestone. This is the first major market where online has overtaken television to become the biggest single medium."
PWC's senior manager Eva Berg-Winters said: "Perhaps surprisingly, a slowing economy has accelerated the migration to digital technology and hence the continuing shift from more traditional forms of advertising to online, which promises return on investment and measurability in a period of instability.
"The only certainty is that this transgression demands fundamental structural change of business models across all industries."
'Meaningless'
But Lindsey Clay, Thinkbox marketing director, said: The internet is a fantastic technology and home to many different marketing activities that do different things.
"As such, it is interesting but meaningless to sweep all the money spent on every aspect of online marketing into one big figure and celebrate it.
"If press added their classified and display advertising revenues together (as the IAB has done with online classified and display advertising, plus everything else) then that figure would be bigger than both TV and online."
The news comes after ITV audiences dropped to an all-time low during August: ITV audiences drop to an all-time low
Media share of market, first half 2009
Internet: 23.5 per cent
Television: 21.9 per cent
Press display – 18.5 per cent
DM: 11.5 per cent
Press classified: 11 per cent
Outdoor: 4.8 per cent
Directories: 4.4 per cent
Radio: 3.3 per cent
Cinema: 1 per cent
Only sector to grow
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