Wiretapping has traditionally been the domain of law enforcement agencies and government spies, but now internet giant Google is facing allegations that it too engages in the practice – illegally – by gathering data from Gmail users and using it to show them targeted advertising.

The claims against Google date back over several years, stemming from a variety of cases, and have now been merged into two separate lawsuits alleging that the company goes too far when it collects data from users of services like Gmail and Street View. Now, following a ruling by a California judge yesterday that rejected Google’s request to have the case dismissed, the search engine giant finally looks set to be dragged before the courts to explain its action.

In yesterday’s ruling, Judge Lucy Koh of the United States District Court judge in San Jose, Calif., rejected Google’s protest that Gmail users, and those who communicate with them, had consented to having their emails read for the purposes of targeted action. The ruling paves the way for a class-action lawsuit against Google.

In its request, Google had insisted that that those who use email services “cannot be surprised if their communications are processed by the recipient’s ECS [electronic communications service] provider in the course of delivery,” just as we shouldn’t be surprised if they sent a letter and it was opened by the recipient’s assistant.

As the New York Times reports, Google’s argument seems to be based on a provision within the Wiretap Law that allows companies that facilitate communications like e-mail to monitor the contents of them – but this provision is only intended for quality-control purposes, such as blocking spam. The plaintiffs therefore argue that ‘wiretapping’ people’s emails to serve ads doesn’t really constitute quality-control, and it seems that Judge Koh agrees.

“Google’s alleged interceptions are neither instrumental to the provision of email services, nor are they an incidental effect of providing these services,” Koh ruled. “The Court therefore finds that Plaintiffs have plausibly alleged that the interceptions fall outside Google’s ordinary course of business.”

The wiretapping claims are just the latest in a string of privacy accusations leveled against Google in recent years. The company is faced with the delicate balancing act of complying with consumer data protection laws, whilst making its main advertising business as profitable as can be. It’s currently on the defensive, arguing that such laws are stuck in the past and have failed to keep up with technological innovations.

Google also asserts that email scanning is done by algorithms and that therefore, no humans ever read people’s email, but even though that’s almost certainly the case, yesterday’s ruling could well prove to be critical for email service providers. Whilst Google has previously avoided major penalties for privacy violations, the Gmail case could be a very different story. Gmail is used by almost half a billion people around the world, and if the lawsuit against it is certified as a class-action it could result in enormous fine. The case could also have consequences for the likes of Microsoft and Yahoo as well, as these also gather up user’s data to serve ads via their own networks.