Large losses of Arctic ozone occur during winters with cold, stable stratospheric circulations that result in the extensive occurrence of polar stratospheric clouds (PSCs). Reactions on the surface of PSCs lead to elevated abundances of chlorine monoxide (ClO) that, in the presence of sunlight, destroys ozone. Here we show that PSCs were more widespread during the 1999/2000 Arctic winter than for any other winter in the past two decades. We have used three fundamentally different approaches to derive the degree of chemical ozone loss from ozone sonde, balloon, aircraft and satelite instruments. We show that the ozone losses derived from these different instruments and approaches agree very well, resulting in a high level of confidence in the results. Chemical processes led to a 70% reduction of ozone for a ~1 km thick region of the lower stratosphere, the largest degree of local loss ever reported for the Arctic. The chemical loss of ozone in the total column amounted to about 100 DU by the end of the winter. This total column loss was balanced by transport, resulting in relatively constant total ozone between early January and late March, which is in contrast to the climatological increase of the total ozone column during this period, that is observed during most years.