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.

In online advertising, where companies are paid according to the amount of visitors that are delivered to a particular site, such technologies can be used to drive up the advertising revenues of the host sites or by companies seeking to artificially inflate the advertising spend of their competitors.

Experts have estimated that up to 20 per cent of fees in certain advertising categories are generated by such click fraud.

The scam also opens the way for further crimes. In 2004, a California man created a software program that he claimed could let spammers defraud Google out of millions of dollars in fraudulent clicks. Authorities said he was arrested while trying to blackmail Google for $150,000 to hand over the program.

Now, some operators are reportedly using low-cost labour in countries such as Mexico, India and China to set up "click factories", to repeatedly access targeted ads.

"There is no doubt that click fraud is a worry and that a lack of information adds to the problem," Mr Chace said.

Thomson said in a statement that the internet is "an opaque medium: the growth is in search-based ads, which the advertising industry cannot monitor, whilst internet display ad revenues actually seem to have fallen at the end of 2005.

"In the longer term we are very conscious that as this medium continues its explosive growth, there is a lack of marketing spend transparency."

Analysts say that worries over click fraud could explain why companies in sectors such as "fast moving consumer goods" (FMCG) have been relatively slow to embrace the trend towards internet marketing.

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