The U.S. Justice Department announced Wednesday that Google Inc. will pay $500 million to settle government charges that it has shown illegal ads for online Canadian pharmacies in the United States.

The fine, which the Justice Department said is one of the largest such penalties ever, covers revenue that Google earned from the illegal advertisers and revenue that the Canadian pharmacies received from United States customers.

As part of the settlement, Google acknowledged that it improperly aided Canadian pharmacies that operate illegally by failing to require a prescription or selling counterfeit drugs in advertising through its AdWords program.

The investigation was first revealed in May, when Google said in a government filing that it set aside $500 million for the potential settlement of a Department of Justice investigation into its advertising practices. The move decreased its quarterly profit by 22 percent.

Google has said in the past that regulating these pharmacies on its site is a cat-and-mouse game, because when it introduces rules to prevent them from advertising, they find new ways to appear on Google.

Websites are liable for ads on their sites from advertisers that break federal criminal law. Google said, in a statement Wednesday, “We banned the advertising of prescription drugs in the U.S. by Canadian pharmacies some time ago. However, it’s obvious with hindsight that we shouldn’t have allowed these ads on Google in the first place. Given the extensive coverage this settlement has already received, we won’t be commenting further.”