The websites of UK media outlets such as the BBC and The Times attract far more visitors from overseas than they do from within Britain and Northern Ireland.
The findings from internet analyst comScore suggest advertisers need to devise international strategies. Advertising analysts predict that worldwide online advertising spend will overtake radio advertising in 2008.
The Financial Times website, ft.com, has an overwhelmingly overseas audience, with 85 percent of visitors coming from outside the UK.
Bob Ivins, managing director of comScore Europe, said: "The internet allows consumers to view relevant content at any site from anywhere in the world. It is therefore critical for publishers and advertisers to understand their global franchise - not just their UK audience."
The BBC had the largest number of UK visitors of the media websites surveyed by comScore, with 17.9 million unique visitors in April - but this was just 39 percent of its traffic, with total worldwide visitors amounting to 46.4 million.
The Guardian's website had the highest visitor numbers among the sites run by UK newspapers, with 3.2 million UK visitors and 7.7 million from overseas - 58 percent of the total. The Financial Times had slightly more than 2 million visitors from abroad, but just 305,000 from the UK.