A local neighbour raised the alarm after she found the bright yellow case marked "radioactive" on Thursday morning outside the secondary school.

Police cordoned off the area and recovered the suitcase intact, a Madrid police spokeswoman told AFP.

Spain's nuclear safety agency (CSN) issued an alert on Wednesday, a day after the material went missing, presumed stolen.

It warned the public of the dangers of opening the suitcase, saying it contained "two sources of low-level radioactivity".

The material could "pose risks if it is opened or if the equipment is destroyed," the CSN warned.

The case was part of a kit for geologists to measure the density and humidity of the earth.