Google is being sued by an American private investigator who claims a glitch in the search giant’s auction-based advertising program is leading to customers being charged without their consent.

David Almeida joined the AdWords scheme in November 2006 and during the sign-up process entered the amount he was prepared to pay for each keyword or search phrase beside which he wanted to advertise his private investigation business.

However, his lawyers claim that a second optional box, which bids for adverts on third-party sites, will post bids if left blank. Users have to enter zero to not place a bid.

Almeida’s lawyer, Brian Kabateck, alleged that the most vulnerable Google customers would be the “unsophisticated advertisers”.

“The sophisticated advertisers will know better, will know how to do it,” Kabateck added.

“These are the little guys that don’t have money to lose on a programme like this.”

Google said it had not been served with the lawsuit and declined to comment further.

The ad search company reported revenues of $5.19bn for the first quarter of 2008, the majority of which comes from its AdWords and AdSense advertising programmes.

Kabateck, who filed the lawsuit in California, has filed a series of consumer rights lawsuits including a successful 2006 case against Google for click fraud.