Police and pollution experts were playing down the risks of contamination from the depleted uranium, which has a wide variety of industrial uses and is found in armour plating and shell cases.
The HMIP said the farm at Chelveston had been used as an illegal dump and the drums containing the metal resulted from fly-tipping.
The incident came to light when the farmer sold some of the drums to a scrap metal dealer in Sheffield who called in experts after suspecting the identity of the metal.
A spokesman for the HMIP said: "We are investigating to see where this material has come from. We have never had uranium dumped in this way before and we are checking with British Nuclear Fuels to see if they can suggest sources."
Police described the drums, containing about 50 kilogrammes of uranium, as "low-grade radioactive waste". The metal is toxic but passers-by would be extremely unlikely to have come into direct contact with it, officers said.
A police spokeswoman said: "The contaminated area, about five square metres, has been cordoned off at a safe distance. There is no danger to anyone outside the cordoned area. People have absolutely no need to worry unless they have been in contact with the waste material."Reuse content