SHANGHAI — A new study by the World Bank and a Chinese government research organization warns that the country’s economic growth is likely to diminish over the next few decades unless China alters its development model and rethinks the role of government in managing the economy.

The report, called “China 2030,” and produced by the bank and the Development Research Center, the government research organization, calls on Beijing to complete the transition to a market economy, scale back the power of state-owned companies, encourage private enterprise and confront rising inequality and environmental degradation.

As remarkable as China’s growth has been over the past three decades, the study suggests that the country’s development pattern has been uneven and is unsustainable. The government should make significant changes, it says.

“The case for reform is compelling because China has now reached a turning point in its development path,” Robert B. Zoellick, president of the World Bank, said in a speech in Beijing on Monday, when the report was released. “Managing the transition from a middle-income to a high-income country will prove challenging; add to this a global environment that will likely remain uncertain and volatile for the foreseeable future, and the need for change assumes even greater importance.”