SHANGHAI  The former chairwoman of one of China’s biggest dairy producers pleaded guilty on Wednesday to selling tainted powdered baby formula and acknowledged for the first time that the company knew of the problem months before alerting local officials to what has become one of the country’s biggest food-safety crises, according to the state-run news media.

The former executive, Tian Wenhua, is one of the highest ranking corporate executives to go on trial in China and could face life imprisonment or even the death sentence if convicted.

Since September, when the scandal became public, investigations have shown how widespread the problem of tainted milk is in China, with watered-down milk being doctored with a chemical used in plastics and fertilizer to falsely raise its protein count. The chemical, melamine, can cause kidney stones and other ailments, and the tainted formula sickened nearly 300,000 children and killed 6.

The government has accused Sanlu and other big Chinese companies of failing to monitor the quality of their powdered baby formula, and in some cases covering up knowledge that their products contained high levels of melamine. The scandal, which follows others in China’s food and drug industries, has devastated the country’s dairy industry, prompted global recalls of suspect food products, and forced the Chinese authorities to try to demonstrate a new seriousness in enforcement.