"For years scientists have suspected that rats have been becoming wary of traditional raticides," Xinhua news agency said on Sunday. 

"The rat-use contraceptive has passed official appraisal and will be produced in batches this March." 

It said the liquid contraceptive could be used to lace peanuts and other foods used as bait. 

The government has already halted production of one common rat poison, tetramine, after it became the weapon of choice in a series of murders.

No harm to humans

Xinhua quoted Jiang Guangzao, an expert with the Sichuan Provincial Institute of Plant Protection, as saying the contraceptive was the most economical way to fight rats.

"And it poses no harm to humans," he added. 

Last month, China announced a campaign in southern Guangdong province to try to wipe out rats and cockroaches amid fears of a new outbreak of the flu-like viral disease SARS.

City anti-rat campaigns have been launched in the past, with rat catchers being paid bounty per rat tail produced - a scheme which has led to dead rats being brought in from the countryside to boost the rat catchers' purse.