(Reuters) - China has put in place rules to protect private investors and expand opportunities in infrastructure and utility projects, the country's top planning agency said on Tuesday, as the government seeks help revitalizing a stuttering economy.

To encourage private investors the National Development and Reform Commission (NDRC) has opened up a number of major infrastructure projects that had previously been off-limits, officials told a news conference.

Private investors can build projects in energy, transportation, water and environmental protection and urban utilities through franchises, according to rules issued by the planning agency.

Contracts would be based on build-operate-transfer (BOT) models. The planning agency pledged to "protect the legal interests of social capital and guarantee stability and continuity of franchising operations".

The government will encourage banks to provide syndicated loans for such franchise projects, and let policy banks offer "differentiated" credit support, including loans of up to 30 years, according to the rules due to take effect from June 1.

The projects will be allowed to raise funds via private equity, strategic investment and bonds, according to the new rules.

Private investment is being encouraged in infrastructure as local governments are saddled with heavy debts largely run up during a massive state-led stimulus implemented after the global financial crisis struck.

The government is looking for ways to lift the economy, as growth slowed a six-year low of 7 percent in the first quarter and recent factory surveys indicated continued weakness into the second quarter.