SHANGHAI  The chief of China’s food and product quality agency was forced to resign Monday in a growing scandal over the country’s tainted milk supply, which has already sickened more than 50,000 infants and killed at least three children, according to the state-run Xinhua news agency.

The official, Li Changjiang, is the most senior government official to lose his job in the scandal. His resignation was announced Monday evening, as the government widened its investigation into how an industrial chemical contaminated powdered baby formula and milk products made by some of the country’s biggest dairy companies.

It is one of the nation’s worst food safety scandals in memory, exceeding the troubles of a year ago when China was found to have exported tainted pet food ingredients, toothpaste, seafood and dangerous lead-contaminated toys.

The government has already arrested 19 people suspected of intentionally spiking milk supplies with melamine, an industrial chemical made from coal that is normally used in the production of plastics and fertilizer.