If you want a glimpse of the future of advertising, you can hire a consultant — or you can travel to Britain.

Online advertising is racing ahead in Britain, growing at a roughly 40 percent annual rate, and is expected to account for as much as 14 percent of overall ad spending this year, according to media buying agencies. That is the highest level in the world, and more than double the percentage in the United States.

There are big differences between the advertising markets in Britain and the Unites States. In Britain, much of the advertising is national, while there are strong local and regional ad markets in America. Still, some believe that online advertising in Britain provides somewhat of a roadmap for where online ads in the United States and elsewhere may be heading. “The U.S. is so behind,” said Terry S. Semel, the chief executive of Yahoo, in a recent speech in London. “It’s certainly lagging the U.K. by at least a year or two.”

More than their American counterparts, British marketers seem to have bought into the oft-touted benefits of Internet advertising: that it is easy to track, enormously effective and a relative bargain. In Britain, as Internet ad spending surges, the overall advertising pie is not growing much at all, and traditional media are the ones losing out.

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