The world's worst ever nuclear accident, at Chernobyl in 1986, sent a plume of highly radioactive material into the atmosphere first detected outside the immediate region by atomic workers in Sweden.
Contamination arrived in Britain through light nuclear rain, and radiocaesium-137 was deposited in mainly upland areas of Wales, Scotland and England where it seeped into the peaty soil used mainly to breed livestock.
The European Commission ruled that a maximum level of 1,000 becquerels per kilogram (bq/k) of radiocaesium should be allowed in sheep being reared for meat to safeguard the consumer. Similar guidelines were given for fish affected by contaminated rivers.
A continuous assessment of restrictions on British farms has taken place over the last 23 years and the number affected has gradually been reduced. Animals are checked by hand monitors and if lower levels of radioactivity are found throughout a flock then the farm is considered for release from the restrictions. The monitoring work is undertaken by the Rural Payments Agency in England and Rural Inspectorate for Wales, answering to the Food Standards Agency.
Britain might see the end of all restrictions within a few years, but some nuclear experts believe there should be no agricultural production around Chernobyl for the next two centuries.
The UN Scientific Committee on the Effects of Atomic Radiation believed there could be 4,000 cases of thyroid cancer in children and adolescents who were exposed at the time in Ukraine, Russia and Belarus. A report commissioned by Greenpeace suggested the number of additional deaths due to Chernobyl could be as high as 200,000.