Google shares took more than a 6 per cent tumble this morning as twitchy investors heard reports the US Department of Justice was about to launch a major investigation into the ad giant's business practices.

Reports from "people familiar with the matter" suggest America's Federal Trade Commission has played a role in preparations but is handing over the reins to the DoJ.

It is not clear which of Alphabet/Google's many tentacles the DoJ is particularly interested in.

The FTC last probed Google back in 2013, but the commish cleared Google of biasing its search results, though the giant conceded that it would license mobile patents under FRAND rules and not fling them around in litigation against rivals.

Mountain View has battled regulators on many fronts in Europe, including investigations into its mobile operating system, online ad sales system and comparison shopping service as well as broader privacy probes.

I don't hate US tech, snarls Euro monopoly watchdog chief – as Google slapped with €1.49bn megafine READ MORE

The search and ad giant is still under investigation by the Irish data protection regulator and has already settled an EU case with a payment of €1.49bn to stop action being taken against its online ad broker system.

The Irish Data Protection Commissioner is following up a complaint that Google's Doubleclick is in breach of European's new data protection laws, the General Data Protection Regulation (GDPR).

The accusation centres on allegations that Google routinely leaks private data about users to its ad-matching service.

Google has also been forced to take restorative action to stop a probe into claims it unfairly favoured its own shopping comparison service over those provided by other companies.

And of course that followed a €4.3bn European Commission fine for abusing its Android payment last year to unfairly favour its own search services.

In the US, Google has had an easier ride from regulators – although some potential Democratic presidential candidates, like Elizabeth Warren, have suggested taking tough action against tech giants including Google.

Some Republicans have also complained that Google, Facebook, Twitter and other Silicon Valley darlings are guilty of anti-conservative bias. ®