Police have appealed to the public for help after a canister containing a small amount of radioactive material was stolen from a van over the weekend.
The container was taken from the vehicle in Bacup, Lancashire.
Officers were alerted to the theft in the early hours of Monday after the van driver discovered the material was missing.
The Peugeot van had been parked in New Line, to the south of Bacup.
The stolen canister is in a yellow case, which is one foot long and said to be fairly heavy.
Chief Inspector Russ Procter from Burnley Police said: “It is important that this material is located as soon as possible.
“I would ask the public, especially in the Rossendale area, to keep an eye out for this canister and if they locate it then to call the police, who will come and deal with it.
“I would also appeal direct to the people who have stolen this. They may have no idea what it is that they have in their possession or they may have discarded it somewhere. If that is the case then I would ask them to contact us or call Crimestoppers anonymously.”
The Health Protection Agency is urging anyone coming in contact with the cylinder to leave it where it is and alert the police.
A spokesperson for the HPA said: “The radioactive material, believed to be Iridium 192, will not pose a risk to the public if it remains contained in its heavy lead container marked with the radiation trefoil.
“However, if the material is outside of its protection packaging, anyone who has come into prolonged physical contact, such as keeping it in a pocket, should seek medical assistance.
“If you see this package you should immediately inform the police, do not touch it and keep away.”
Iridium 192 is a radioactive isotope of Iridium with a half-life of 73 days.
It emits beta and gamma radiation and as a result has the potential to cause damage to human cells if they come into contact with it or are exposed to it for prolonged period of time.
It is used in industrial radiography to locate flaws in metal components and also in radiotherapy as a source of radiation to kill cancers.
Anyone with information is asked to call Lancashire police on 999 or call Crimestoppers anonymously on 0800 555 111