Last month it was revealed that someone somewhere was illegally manufacturing a banned ozone-destroying chemical. Now it appears that small factories in China are the source.

According to investigators at the non-profit Environmental Investigation Agency and the New York Times, companies in China are buying a banned chlorofluorocarbon called CFC-11 to make foam insulation for fridges.

Small illegal factories are springing up to supply this demand, and simply move elsewhere if they are discovered and shut down.

Fridge makers told the investigators that they have been buying CFC-11 because it is cheaper than the main legal alternative, HCFC-141b, which is in short supply in China.

Levels of ozone-depleting chemicals in the atmosphere have been slowly declining since the 1990s, after they were phased out under the 1987 Montreal Protocol.

The ozone layer, which protects life by reducing levels of ultraviolet light, was on course to recover fully by mid-century.

But the rate of decline in CFC-11 has halved since 2012, researchers reported in May. Last month, some researchers suggested that some of this change could be down the careless recycling of discarded refrigerators in China .

Now this new investigation suggests it is being made illegally too. This could delay the recovery of the ozone layer by a decade or more.