The dead bird was found by a technician of the Institute for Nature and Forest Conservation (ICNF) on 4 January beneath pine trees where the species is known to nest.
It was close to a spot where an Iberian lynx was found dead last year after also having been poisoned.
Speaking to newspaper Público, Rita Alcazar of the League for Nature Protection (LPN) said the dead bird showed “evidence and body posture consistent with a possible poisoning”.
The same signs were identified in two other dead Iberian Imperial eagles found in Moura in 2015 and Castro Verde in 2013, which were later confirmed as having died from poisoning.
GNR police were called to the scene of this latest incident to collect evidence that has been sent for examination to determine whether or not the eagle was poisoned.
In a statement issued meanwhile by the LPN, the League said “several cases of death of various species by poisoning” have been detected in the Alentejo, with four cases having occurred in December last year.
Ms. Alcazar underlines the impact that the death of Imperial eagles could have on the viability of the species as a whole in Portugal.
In 2015 just 13 couples were mapped, and the species is known to reproduce slowly. It also faces growing threat from poisoning and electrocution by high energy network lines.
In Portugal the Iberian Imperial eagle is classified as being “critically endangered.”