BEIJING -- A southern Chinese province has stopped sending prisoners to labour camps, becoming the first in the country to take steps to phase out the much-criticized system.

The move, which is expected to extend to the rest of China this year, is considered a key step in reforming China's judicial system. Critics have condemned the labour camp system as arbitrary and unconstitutional because it allows police to lock up government critics and other defendants for up to four years without trial.

China has 310 labour camps holding about 310,000 prisoners and employing 100,000 staff, according to state broadcaster CCTV.

Yunnan's top law enforcement official Meng Sutie announced Tuesday that the province will no longer send people to labour camps on the grounds of threatening national security, petitioning by causing unrest and smearing the image of officials, according to Chinese state media and a Yunnan labour camp official.

The province also is suspending labour camp sentences for people charged with other offences, such as drug use and prostitution, Meng said.

Those in the camps will be released after completing their terms, said Meng, who was quoted by the official Xinhua News Agency.

"We believe this is a good thing, and we raise both hands to show our support," said Pu Zhiqiang, a Chinese lawyer who represents Ren Jianyu, a local official sentenced to two years in a labour camp after criticizing a Maoist revival campaign on the Internet.

Ren's case has helped amplify calls to abolish China's labour camps, which were initially set up in the 1950s to detain accused counterrevolutionaries or other critics of the Communist government. The system was later expanded to punish prostitutes, drug addicts and other minor criminals.

Lawyers and human rights activists have condemned the system as outdated and open to abuse, especially in locking away those who criticize officials or government policies.

Those sent to the camps include minor officials such as Ren and private citizens who attacked the once high-flying politician Bo Xilai over his brutal crackdown on organized crime and his promotion of Mao Zedong-era Communist culture. Some were released following Bo's spectacular fall from power in 2012, the country's biggest political scandal in years.

Authorities also sentenced large numbers of Falun Gong adherents to the camps after banning the spiritual group in the late 1990s.

In the past decade, China also has been sending stubborn petitioners -- people seeking to have local grievances addressed -- to labour camps.

Last year, a woman in central Hunan province was sentenced to 18 months in a labour camp for causing a social disturbance after she repeatedly petitioned for harsher penalties for the seven men convicted of abducting, raping and prostituting her 11-year-old daughter. Tang Hui was released within days following overwhelming public opposition. Her case also prompted Xinhua to call for the system to "be swept into the dustbin of history."

In January, the ruling Communist Party Politics and Law Committee head Meng Jianzhu said China would stop handing down labour camp sentences this year. But the proposal must first be approved by China's legislature, the National People's Congress, which will meet in March.

Yan Zhichan, director of the Department of Justice in the southern province of Guangdong, said at the end of January that her province had made preparatory work to end the labour camp system.