Analysts have warned that a lack of transparency surrounding internet advertising figures is leaving the way open to click fraud scams which could be dramatically skewing the rapidly growing multi-billion dollar market.

Calum Chace, the commercial director of Thomson Intermedia, a leading marketing agency, said: "This is a dark area."

The problem, he suggested, stems from a reluctance of Google and Yahoo!, the leading players in the market, to make their detailed data public. That has led to resistance from some advertising agencies to advise clients to enter areas such as paid-search advertising, despite of rapid growth, Mr Chace added.

Fears over what could happen behind the scenes of online advertising campaigns have existed for several years. Click fraud dates back to the early days of the internet when exponents developed software that gives the appearance that a particular link is being clicked.

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