Search giants Yahoo and Google saw their shares plummet after Yahoo warned it was seeing weakness in two of its biggest advertising segments. Sliding revenues from car and financial services advertising meant third-quarter revenues would be at the bottom end of forecasts, said Susan Decker, the chief financial officer, sending Yahoo shares down as much as 15pc and close to their lowest level for more than two years.

"It's a new trend. It's been two to three weeks and we don't know yet if it's an indicator of a broader slowdown," said Miss Decker. "We're seeing it enough to say something. I don't want to overplay it either." In July, Yahoo forecast third-quarter revenue of $1.12bn (£596m) to $1.23bn, but yesterday admitted they would be "at the bottom half of the range".

Her comments ignited concern for other Internet companies who rely heavily on advertising, including search leader Google. Google shares tumbled more than 5pc. Ebay and Amazon both slid around 3pc. Car makers in particular have moved a large portion of their advertising to the Web in the last three years, but industry leaders like General Motors and Ford have been slashing billions of dollars in costs as they grapple with losses in the North American market.

Martin Pyykkonen, an analyst with Global Crown Capital, said: "It would be naive to say that advertisers would continue to pour ahead on online advertising and cut back only on traditional advertising in the face of economic weakness."

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