An environmental pressure group claimed Monday that Chinese factories are illegally using ozone-depleting CFCs, which have recently seen a spike in emissions that has baffled scientists.

The Environmental Investigation Agency (EIA) campaign group said 18 factories in 10 Chinese provinces they looked into admitted to using banned chlorofluorocarbons.

Producers and traders told EIA researchers posing as buyers that the majority of Chinese companies manufacturing foam - in high demand as an insulator in the booming construction sector - continue to use CFC-11 because of its better quality and lower price.

CFCs are chemicals that deplete the ozone layer, the thin gaseous shield that protects life on Earth from dangerous solar rays.

They were banned under the internationally binding 1987 Montreal Protocol and production of CFCs officially ended in developing countries in 2010.

Chinese authorities said the country successfully ended the industrial practice of using CFCs in 2007. The environment ministry in Beijing could not immediately be reached for comment.

A representative from one company cited in the report said the firm sources CFCs from unlicensed factories with "shady" operations in Inner Mongolia and conceals the substance from customs agents.

Others cited in the report said their companies produce the substance themselves, with one source saying its factories can produce 40 tonnes of CFC agents per day.