The hole in the ozone layer over Antarctica is about to start shrinking and will close by 2050, Australian researchers say.

Government scientists in Tasmania said scientific data showed the level of ozone-depleting chlorine in the atmosphere was declining because of the ban on the use of chlorofluorocarbons (CFCs) in fridges and air conditioners, agreed under the Montreal Protocol in 1987.

Dr Paul Fraser, the chief atmospheric research scientist with the Commonwealth Scientific and Industrial Research Organisation, said: "This is big news. We have been waiting for this. We think the trend is definite now. It is very significant."

Dr Fraser said that after the ban was adopted, the atmospheric level of chlorine continued to rise, peaking in 2000. "That was simply due to old refrigerators and old air-conditioners in cars," he says.

Since then, the level had stabilised and was now declining, albeit slowly, Dr Fraser said. He predicted that the hole in the ozone layer would contract steadily from about 2005 and disappear by mid-century, although the ozone would be vulnerable for a decade.

Under the Montreal Protocol, developing countries committed themselves to halving consumption and production of CFCs by 2005 and achieving an 85 per cent cut by 2007. The hole in the ozone layer, which protects Earth from the Sun's ultraviolet rays, was first detected nearly 30 years ago. It was then three times the size of Australia, itself covering nearly 3 million square miles.