An Easter egg hunt in south-western Germany took a worrying turn on Sunday when two men discovered a vial labelled "Polonium 210" in a garden, triggering an emergency response as authorities tested for the potentially lethal radioactive substance.

District fire chief Andy Dorroch said initial on-site measurements were carried out to detect radioactivity, but all of them came back negative. He added that the two men were unharmed.

The discovery led to a large-scale operation involving the fire brigade and police in the town of Vaihingen an der Enz, north-west of Stuttgart.

It remains unclear whether the 50-millilitre vial actually contained polonium 210.

The fire brigade will secure the vial in accordance with safety precautions, the fire chief said.

Reports said the area around the site where the bottle was found was cordoned off.

According to the Federal Office for Radiation Protection (BfS), the chemical element polonium is particularly dangerous if inhaled or absorbed through the skin via open wounds.