Health authorities in Britain have recalled two lots of counterfeit Cialis tablets after fake versions of the erectile dysfunction drug were sold in at least one pharmacy there.

The British Department of Health’s regulatory office said initial tests of the fake tablets showed they did not pose an immediate health risk to patients.

The problem in Britain seems to be an isolated one, said Lacy Fitzpatrick, spokeswoman for Bothell-based ICOS Corp., which makes Cialis with Eli Lilly & Co., its pharmaceutical partner. “We haven’t had a similar situation in the U.S.,” Fitzpatrick said.

British health officials also said they had no concerns about the quality of genuine Cialis tablets in that country.

It’s easy to find fake versions of Cialis - along with its rivals, Viagra and Levitra - being sold under a variety of names on shady Web sites or via spam e-mails. To combat that problem, Eli Lilly, which controls sales and marketing of Cialis in Asia, has helped police in several of that region’s nations find and bust makers of counterfeit tablets. Viagra’s maker, Pfizer Inc., has moved to sue a number of online pharmacies.

The discovery in Britain stands out, however, because the counterfeit pills made it into the legitimate drug supply. The government said this was the first incident of a fake drug found in the legitimate supply chain since 1994. According to U.K. media accounts, the fake tablets were discovered after a patient noticed one of the tablets from the package he bought was crumbling apart.