The substance was discovered yesterday sealed in a small metal box with a radioactive warning sticker.

Police and Office of Atoms for Peace (OAP) officials inspected the item in the two-storey building for any leakage and sealed off the area as a safety precaution.

The building was previously leased to a South Korean company, it was reported.

At 4.20pm OAP officials revealed the substance was iridium-192, a radioactive isotope, and there was no trace of radioactive leakage at the scene.

Police collected evidence and passed the item onto OAP for its safe transport.

Acting city police chief Lt-General Sanit Mahathaworn inspected the scene later in the evening.

The iridium-192 expired in 1995. According to an expert, iridium-192 is used in gamma ray photography and to detect pipe cracks or leakages.

The International Atomic Energy Agency defines iridium-192 as a category two radioactive substance, meaning it can kill people in close proximity to it within hours to days.