US Edition A record £1.75bn - nearly a quarter of the total ad market - was spent online in the first half of this year, compared with £1.64bn for TV, the study said.
It is thought that the UK is the world's first major economy to pass the media milestone.
Internet spending grew by 4.6pc during the period compared to the previous year and now makes up 23.5pc of the market. TV advertising fell by 17pc, making up 21.9pc of total spend.
The study, carried out by the Internet Advertising Bureau (IAB) and PricewaterhouseCoopers, showed overall advertising fell by 16pc in the period as firms slashed costs in the recession.
IAB chief executive Guy Phillipson said he thought online advertising still had room for growth.
"We could see absolutely see it grow to being a 30pc medium [of ad share spend], to go past £4bn to even £5bn annually," he added.
Online advertising is classified as email campaigns, classified adverts, display ads and search engine websites such as Google.
The IAB data showed that 60pc of the online advertising was spent on search engines, with 22pc on online classified ads.
Justin Pearse, editor of industry website New Media Age, told the BBC that the tough economic times had led to a significant fall in TV advertising spending, which saw it being overtaken about a year earlier than most had expected.
Guy Phillipson, chief executive of the Internet Advertising Bureau, said online display advertising - such as banners - had "performed notably well against its peers in TV, print and radio".
However, Thinkbox, the UK TV marketing body, said the figures were not comparing like with like.
Lindsey Clay, Thinkbox's marketing director, said: "Online marketing spend is made up of many things including email, classified ads, display ads and, overwhelmingly, search marketing. They should be judged individually.
"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."
She said that television advertising remained the most effective advertising medium "pound for pound".
Press display advertising was the UK's third biggest ad market, with £1.38bn of spend, or 18.5pc of the total.
Britain remains the world leader in terms of market share for online advertising, due to the use of online networks to place advertising, the availability of fast and cheap broadband and the popularity of new formats such as video adverts.