According to local news reports, the deputy head of China's State Intellectual Property Office (SIPO), Zhang Qin, said the patent licence originally granted in 2001 was invalidated on the advice of experts because of inadequacies with technological disclosures in Pfizer's patent document.
The SIPO reviewed the case in response to complaints from more than 10 Chinese pharmaceutical companies.
The patent was revoked by the SIPO in July on the grounds that Pfizer was in breach of intellectual property law because it failed to accurately explain the uses of the pill's key ingredient, sildenafil citrate.
The US firm insists that at the time of filing there was no requirement for that data, and that to invalidate the patent on that basis is a flawed, retroactive judgment.
Zhang also told a news conference that Viagra had not been granted a patent in many other countries "so China was not alone in making such a decision".
Pfizer has said it will appeal the decision. It has been marketing Viagra in China since it was awarded a patent there in September 2001 and while it was not prepared to disclose sales there, said it was a minor market for the drug. Worldwide Viagra sales were $1.8 billion (€1.5bn) last year.
China has offered full patent protection to foreign companies since January 1993, and more limited protection since 1986. However, many companies did not take advantage of this earlier legislation, and there is a legal loophole that allows Chinese companies to copy many products without a licence.
The law in question is the Medicine Administrative Protection Statute of 1992, which says that holders of foreign pharmaceutical patents taken out between January 1986 and January 1993 could apply to the Chinese government for protection. As long as these products had not yet been sold in China, the government would grant a period of seven-and-a-half years during which Chinese companies were not allowed to copy them.
Pfizer's use patent in China was filed after 1993, so it should be subject to World Trade Organisation intellectual property regulations - although it is acknowledged that these still leave considerable discretionary powers to member countries.