The depletion of the ozone layer shielding Earth from damaging ultraviolet rays has reached an unprecedented low over the Arctic because of harmful chemicals and a cold winter, the UN weather agency warned today.

The World Meteorological Organisation said the Earth's fragile ozone layer in the Arctic region suffered a loss of about 40% from the start of winter until late March, exceeding the previous seasonal loss of about 30%.

The Geneva-based agency blamed the loss on a build-up of ozone-eating chemicals once widely used as coolants and fire retardants in a variety of appliances and on a very cold winter in the stratosphere, the second major layer of the Earth's atmosphere.

Arctic ozone conditions vary more than the seasonal ozone "hole" which forms high in the stratosphere near the South Pole each winter and spring.