Scientists have discovered a hole five times the size of Germany in the ozone layer above the Arctic, allowing harmful ultraviolet radiation to hit northern Canada, Europe and Russia this spring.

The 2 million square kilometre Arctic hole is similar to the hole over the Antarctic, researchers write in the journal Nature, released yesterday.

They say 80 per cent of the ozone was lost about 20km (13 miles) above the Arctic and that a prolonged spell of cold weather – when chlorine chemicals which destroy ozone are at their most active – was to blame.

"Why [all this] occurred will take years of detailed study," said Michelle Santee from Nasa's Jet Propulsion Laboratory, part of the group that monitored the hole from space using satellites. "It was continuously cold from December through April, and that has never happened before in the Arctic."