The ozone layer over Britain has suffered the worst damage ever recorded, due to a combination of pollution and intense cold at high altitude.
The measurements, at Lerwick in Shetland and Camborne in Cornwall, surprised and alarmed scientists. They had forecast that the ozone should be on the verge of starting a recovery after decades of deterioration, thanks to international treaties curbing emissions of the industrial gases and solvents which destroy it.
The ozone layer absorbs much of the harmful ultraviolet B radiation in the suns rays. High levels of these UVB rays can cause skin cancer and cataracts in humans, and can affect the environment, including crops, wild plants and sea plankton.
On Tuesday the Met Office ozone recording station at Lerwick, one of only two in the country, recorded 195 Dobson Units - a measure of the total quantity of ozone in the atmosphere immediately above. It was the first time there had been a reading below 200 in Britain. The same low levels were recorded as far away as Cornwall, where the measure was 206 Dobson Units. That was the lowest level recorded there since it was set up 17 years ago.
At Lerwick, this February's readings have, overall, been well below the month's long term average - but they have been especially low in the past 10 days, reaching a peak with Tuesday's record.
Dr Joe Farman, the British Antarctic Survey scientist who first discovered the ozone hole over the South Pole, said: ``This is certainly significant, and shows the problems haven't gone away.
``We've warned that things would get worse before they start to get better, but it's impossible to make any precise predictions. With the very cold winters we have been getting at high altitude, the ozone loss could well accelerate.''
Ozone is a gas found at very low concentrations in air. It absorbs UVB in the stratosphere - the upper atmosphere above 35,000 feet - and its concentration there fluctuates with changing weather patterns and season.
But there has been a gradual, global decline in this high level ozone for several decades, thanks to the rapidly growing use of chemicals containing chlorine and bromine used in refrigeration, air conditioning and dry cleaning. These CFCs (chlorofluorocarbons) and other compounds escape slowly into the stratosphere.
The first severe and rapid ozone destruction was detected above the Antarctic in the mid-1980s. Each Spring in the southern hemisphere an "ozone hole" opens up there - a continent-sized patch of stratosphere in which half or more of the ozone has been lost.
This is caused by a complex cycle of chemical reactions, driven by sunlight, which take place on the surface of high altitude ice clouds.
Scientists have been debating whether similar ozone holes could open up over the Arctic, covering populated regions in northern Europe, Russia, Alaska and Canada.
They have monitored substantial ozone losses in recent northern hemisphere Springs. Last year's was among the worst ever.
Dr Farman said the ozone destruction taking place this Spring over the northern hemisphere could be worse still. ``We have to get it through to the politicians that we have not yet cleaned up this stuff,'' he said.
Man-made global warming, caused by a build-up of heat trapping gases, appears to be exacerbating the ozone loss. While temperatures rise in the lower atmosphere, those in the stratosphere drop. This makes sustained ozone destruction more likely, because it helps the formation of the high level ice clouds and allows them to exist for longer.
Since 1987, a series of international negotiations have imposed tighter controls on the production of ozone-destroying chemicals.
The latest agreement, under this Montreal Protocol treaty, took place in Vienna last December. But environmental organisations like Greenpeace and atmospheric scientists like Dr Farman say the rate of progress is still too slow.