Britain's advertising watchdog is receiving a growing number of complaints from people who believe that they are being misled by internet search advertising campaigns, the fastest growing area of the surging online ad market.

Two complaints being looked at by the Advertising Standards Agency involve Google.

The market leader in search also faces a potential landmark lawsuit from the Australian consumer watchdog over allegedly engaging "in misleading and deceptive conduct" in its search-ad business.

Experts say that the Australian case has the potential to hinder severely Google's massively lucrative search-advertising business, the source of the vast bulk of its profits, by making the company liable for vetting aspects of ads, such as whether trademarks are being violated, before they are included on its site.

Google also faces a $1 billion (£490 million) lawsuit from Viacom, the media giant, which alleges that YouTube, the video sharing website bought by Google for $1.65 billion last year, has not done enough to tackle piracy.

Another new action emerged today, when it was reported that Beijing Guge Science and Technology, a Chinese company, is suing the search engine over its Chinese name — "Gu Ge" — which it says is too much like its own and is harming its business.

The ASA said that both UK search-ad cases — which involve ads that make allegedly misleading claims from Sony, the electronics group, and Virgin Media, the cable TV company — are being assessed to determine whether they warranted full investigations.

The ASA does not have the power to bring legal cases but is charged with ensuring that advertising in the UK meets certain standards.

The UK watchdog said that it had received more than 40 complaints since June last year over adverts triggered by internet searches.

That figure made up only a tiny fraction of the 25,000 compaints handled by the ASA over the period, but an ASA spokesman said that internet advertising was "rocketing up" the watchdog's agenda as the public become more aware of the area and advertising budgets migrate online.

Paid-search advertising in the UK was worth £1.2 billion last year, according to the Internet Advertising Bureau, more than half the online total and up more than 50 per cent on 2005.

The Australian Competition and Consumer Commission's case against Google alleges that some sponsored search results are misleading.

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