"The meteorological conditions observed so far could indicate that the 2009 ozone hole will be smaller than those of 2006 and 2008 and close to that of 2007," said the UN agency in a statement.

The hole in the layer over the Antarctic was discovered in the 1980s. It regularly tends to form in August, reaching a maximum size late September or early October before it fills again in December.

The size is dependent on weather conditions.

This year, the hole began forming "earlier than before," said WMO's expert on the ozone Geir Braathen.

On September 16, it stood at 24 million square kilometers, he said.

In 2008, the maximum reached was 27 million square km while in 2007, the maximum was 25 million square km.

Experts have warned that the damage to the ozone layer, which shields the Earth from harmful ultra-violet rays, is so bad that it will only attain full recovery in 2075.

Ozone provides a natural protective filter against harmful ultra-violet rays from the sun, which can cause sunburn, cataracts and skin cancer as well as damage vegetation.

Its depletion is caused by extreme cold at high altitude and a particular type of pollution, from chemicals often used in refrigeration, some plastic foams, or aerosol sprays, which have accumulated in the atmosphere.

Most of these chemicals, chlorofluorocarbons (CFCs), are being phased out under the 1987 Montreal Protocol,