The man, named only as Johannes M., was helping to dismantle a recycling plant in Karlsruhe in southwest Germany. He told police that he was merely trying to expose the weak security of the plant but detectives are investigating whether he was trying to sell the plutonium to terrorists or aspiring nuclear states.

The smuggled quantity appears to have been small — a tiny phial that could be hidden in a rubber glove — and well below the mass needed to produce a bomb. Even so, security at the plant was so poor that the man could have made several smuggling trips.

A spokesman for the Stuttgart Environment Ministry said last night that every worker in the plant had to take off his safety suit at the end of the shift. He or she then takes a shower using an especially strong soap and can put on private clothing only if a geiger counter registers zero radiation. Some workers have to pass through five separate controls.

Plainly, however, the system does not work. Herr M. carried the tube out in his trousers and kept it at home. When a routine urine check showed that he had high levels of radiation in his body, he told his wife to wrap the tube in a glove and bury it under a hedge.

A spokesman for the Greens, the junior partner in the German Government, said that security at the plant seemed to be about as thorough as in a chip shop.

Nowadays German nuclear waste is recycled only in Sellafield and at La Hague in France; the Government is committed to abandoning atomic power within the next 30 years and is starting to wind down reactors.

The Karlsruhe plant was an attempt to solve Germany’s nuclear waste problems on its own soil. Between 1971 and 1990 it recycled spent rods and since then has maintained two huge reinforced ponds containing a kind of atomic soup — 70,000 litres of dangerous chemicals. The mixture includes 16.5kg of pure plutonium and half a tonne of uranium. The containers are steel, surrounded by concrete five metres thick.

An automatic paddle stirs the soup to ensure that it does not settle on the floor of the pond, gain a critical mass and set off a chain reaction.

The original plan was to send the brew to Belgium, but it was deemed too dangerous to transport. Instead, a vitrification plant is to be built in the area so that the radioactive material can be encased in glass. In the meantime the plant — like several reactors in Germany — is being slowly dismantled and security standards have slipped.

Helmut Hübner, a spokesman for the plant, said yesterday that there were 40 registered technical failures there last year alone. In 1999 several workers were irradiated when a ventilation shaft malfunctioned and a few years earlier some radioactive material disappeared.

“I could not believe that there could be such extremely lax security here in the middle of Germany,” Susanne Ochse, a nuclear expert for the Greenpeace protest group, said.

Until now most plutonium theft has been from Russian plants.