The finding prompted the British scientist who discovered the hole in the Antarctic ozone layer to warn Western countries against complacency about the effects of treaties such as the 1989 Montreal Protocol, which bans the use of chlorofluorocarbons (CFCs) and other chemicals that destroy ozone.
"Politicians are very taken with the idea that they have cured the ozone layer because of the Montreal Protocol, but they don't realise that it's not like switching a button," said Joe Farman of the European Ozone Research Co-ordinating Unit in Cambridge.
The ESA said measurements taken in the Netherlands showed local ozone levels were about two-thirds below the norm for this time of year. The readings show the depth of the layer over northern Europe is 165 Dobson Units. The "safe" value is taken to be 200 DU or more.
A gradual thinning of the ozone layer caused by emissions of damaging man-made chemicals such as CFCs has increased the occurrence of skin cancer and other illnesses related to over-exposure to ultraviolet rays. CFCs survive in the stratosphere for many years, accelerating the breakdown of ozone there.
However, Mr Farman said the present hole over Europe, which has lasted for three days and is larger than France and Spain, is probably less harmful because of the time of year. "The sun is low in the sky, so you're getting less UV anyway than if it was summer and you were sunbathing."
Today is the end of a five-day international meeting in Peking, intended to speed the phasing out of ozone-eating gases. Non-governmental organisations, including Greenpeace, have criticised the UN for moving too slowly to ban the use of CFCs and related chemicals in developing countries.