Over huge areas of Europe, North America and Asia in the northern hemisphere and over southern Australasia, Latin America and Africa, the layer would be back to pre-1980 levels by 2049, the agencies said.

This was five years later than forecast in the last major scientific report in 2002.

The agencies' message came in an official summary of a report by 250 scientists to be issued next year on the effects of the 1987 Montreal Protocol, which committed signatory nations progressively to ban the use of ozone-harmful products.

"The early signs that the atmosphere is healing demonstrate that the Montreal Protocol is working," said Achim Steiner, executive director of UNEP.

"But the delayed recovery is a warning that we cannot take the ozone layer for granted and must maintain and accelerate our efforts to phase out harmful chemicals," he said in a statement issued in Geneva and Nairobi.

Over Antarctica, where so-called "ozone holes" have grown over the past 30 years, recovery was likely to be delayed until 2065, 15 years later than earlier hoped.

"While these latest projections of ozone recovery are disappointing, the good news is that the level of ozone-depleting substances continues to decline from its 1992-94 peak in the troposphere and the 1990s peak in the stratosphere," said WMO secretary-general Michel Jarraud.

The ozone layer blocks harmful ultra-violet rays and holes in it have been blamed for increased risk of skin cancer and cataracts in humans. It may also harm crop yields and sea life, according to researchers.

Its depletion is caused by the chemical action of chlorine and bromine released by man-made chlorofluorocarbons (CFCs), which are used in aerosol sprays and cooling equipment, like refrigerators and some air-conditioning systems.

WMO and UNEP said the revision of the ozone recovery dates over parts of the northern and southern hemispheres was mainly due to signs that increased amounts of some types of CFCs not immediately banned under the Protocol were being used.

Another reason for the change in the forecast was that estimates had been increased of future production levels of HFCF-22, a CFC substitute, which although much safer, still causes some ozone depletion.