By Richard Black
BBC environment correspondent
The coming weeks could bring the most severe thinning of the ozone layer over northern Europe since records began.
A healthy ozone layer is important to all northern species
The conditions are being driven by unusual weather in the high atmosphere above the Arctic, says the European Ozone Research Coordinating Unit.
The stratosphere, where the ozone layer lies, has seen its coldest winter for 50 years; there have also been an unusually large number of clouds.
These factors hasten the rate at which man-made chemicals destroy ozone.
"The meteorological conditions we are now witnessing resemble and even surpass the conditions of the 1999-2000 winter, when the worst ozone loss to date was observed," said Dr Neil Harris, from the Cambridge University-based unit.
Ozone is a molecule that is composed of three oxygen atoms. It is responsible for filtering out harmful ultra-violet radiation (less than 290 nanometres) from the Sun.
The molecule is constantly being made and destroyed in the stratosphere, which exists from about 10km to 40km above the Earth.
In an unpolluted atmosphere, this cycle of production and decomposition is in equilibrium.
But a number of human-produced chemicals, such as the chlorofluorocarbons (CFCs) used as refrigerants, in aerosol sprays, as solvents and in foam-blowing agents, have risen into the stratosphere where they are broken down by the Sun's rays.
Chlorine atoms released from these chemicals then act as catalysts to decompose ozone.
At the moment, the area where the ozone layer is particularly thin is constrained by winds, which to some extent isolate the Arctic from the rest of the global climate system.
Scientists say this natural barrier will break down in the coming weeks, and the low ozone area will spread southwards over northern Europe, including the UK.
This will mean more of the Sun's ultra-violet rays reaching ground level, potentially increasing the risk of skin cancer.
The incidence of malignant melanoma, the worst kind of skin cancer, is rising; but to what extent that has been caused by decades of ozone depletion is far from clear.
"We will watch the development closely from day to day, and will inform the public and our authorities if the situation becomes worrying," said Dr Harris
The use of ozone-depleting chemicals is now restricted by an international treaty, the Montreal Protocol; but it may be half a century before levels of these chemicals have fallen sufficiently in the atmosphere to allow the northern ozone layer to be fully repaired.