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 the men found a small white plastic bottle with a red cap and called the emergency services. 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.

"We are assuming that this is indeed the substance in question," Dorroch said. He noted that the name was "not just scribbled on by hand, but was clearly and officially labelled."

The vial also weighed an estimated 200 grams, which would correspond with a relatively heavy substance like polonium, he said.

However, initial on-site measurements to detect radioactivity came back negative, he said.

Officials from the Environment Ministry collected the vial in Vaihingen an der Enz in order to analyse its contents, police said.

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.

The Kremlin critic and former intelligence officer Alexander Litvinenko died in London in 2006 following an attack involving polonium 210.