BEIJING (REUTERS) - Chinese officials must not illegally imprison people seeking justice through petitions, a top central government body said on Wednesday, casting a spotlight on the country's widespread network of black jails.

The system of petitions dates back to imperial times as a means for citizens to bring grievances to the attention of government officials.

In practice, few of the cases are ever resolved and petitioners are frequently forced home or imprisoned by local or provincial authorities when they seek to escalate their complaints to higher-level officials.

Government agencies must "resolutely avoid blocking the people from normal petitioning by any means," the ruling Communist Party's Central Committee and the State Council, China's cabinet, said in a statement, the official Xinhua news agency reported.

Petitioners have often been forced into so-called "black jails," where they can be subject to beatings, torture, and food and sleep deprivation.

Similar calls to abolish black jails have gone unheeded in the past, and the statement did not say how the new rules would be enforced.

As part of a landmark package of social and economic reforms enacted early this year, China said it would abolish a controversial system of labour camps - but rights groups say many other forms of extra-judicial detention, including black jails, remain unchecked.

Petitioners commonly seek redress from the government for land grabs, water pollution, corruption and other issues.

The government does not formally acknowledge that black prisons exist.

A system that ranks local governments based on the number of petitioners from their area appealing to Beijing drives local officials to adopt violent means to keep petitioners from lodging their complaints.