The World Meteorological Organisation (WMO) said in a statement that the 28-million-square-kilometre (11-million-square-mile) hole was in "stark contrast" to last year, when it had shrunk to its smallest level in more than a decade and split in two.

"It cannot be predicted with certainty whether the ozone hole will continue to grow during the next few weeks," WMO said.

Last year's recovery had prompted some officials to claim that international efforts to control chemical-based pollutants which destroy the ozone layer were starting to have an impact.

The layer of ozone gas in the earth's upper atmosphere filters harmful Ultra-Violet (UV) rays from the sun, and health officials have warned that excessive exposure to UV can cause skin cancer and other ailments, as well as destroy plant life.

The hole, which is influenced by seasonal climatic conditions as well as pollution, normally reaches its peak in mid to late September and is regarded as a barometer of the overall state of the ozone layer in the rest of the world.

The layer has been eroded by pollutants such as aerosol gases or refrigerants, although these substances are gradually being phased out under a 1987 UN treaty, the Montreal Protocol.

"Complete recovery of the ozone layer will require continuing diligence with the enforcement of international agreements," WMO warned.

Environmental groups have argued that the treaty should be expanded to cover newer chemicals which are used in products ranging from fire extinguishers to cleaning fluids.

In July, NASA, the US National Aeronautic and Space Administration, said that satellite images had signalled that ozone depletion could be stabilising.

Scientists believe that ozone-destroying chemicals -- Chloroflurocarbons (CFC) -- have been decreasing in the lower atmosphere because of less emissions, but not in the upper atmosphere.

"There are indications that it is probably levelling off in the (higher) stratosphere, but that is still to be determined," WMO senior scientific officer Michael Profitt told AFP.

Climatic conditions in the "critically important" upper atmosphere over the Antarctic last year were very unusual according to Profitt, with warmer temperatures helping the ozone layer recover temporarily.

"The variability of the size of the ozone hole depends much more on meteorology than on chemistry," he said.

But Profitt insisted there was a delay in the impact of cleaning the atmosphere, and predicted that efforts to eliminate CFCs would bear fruit in a few decades.

"I think by the year 2050 it will return to pre-ozone hole type levels in the stratosphere," he said, cautioning that the estimate was still tentative.

The Antarctic ozone hole -- which touches the tip of South America and can have an impact in Australasia when it expands -- was first observed by British scientists in the 1980s.

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