Net revenues at Yahoo!, AOL, MSN and Google accounted for 57.4 per cent of ad spend said the eMarketer report, predicting that this would rise to 66 per cent this year.
The dominant position of the 'big four' has been steadily consolidated over the past four years, rising from 47.8 per cent of revenue to 53.7 per cent in 2004/05.
"As traditional marketers move more money online, they look for safety in established, mass-market brands, and portals are that," said eMarketer analyst David Hallerman.
"Other than Google, the large portals are at least ten years old, and all four average 100 million or more unique visitors monthly."
The four largest search players were also predicted to increase their revenues by a third to almost $13 billion during 2007, having already tripled their advertising income from $4.6 billion in 2004.
Within the group, this is being driven by the smaller players, with ad revenues growing at more than a third at AOL and MSN following the change from subscriber to advertiser business models, while Google's stratospheric revenue growth is slowing.
"Industry prognostications aside, the idea of a portal is not going away," said Mr Hallerman.
"A something-for-everyone kind of experience will continue to attract a certain type of advertiser and consumer. Even though that mass-market approach cannot appeal to everyone, let's acknowledge its meat-and-potatoes place on the web."
