Veterinary diclofenac, a medicine for livestock, has been linked to the poisoning and rapid decline of the once common Gyps vultures on the Indian subcontinent.
Numbers plummeted by 97% in just 15 years between 1992 and 2007.
Conservationists fear Europe's birds of prey could face the same fate after the drug was authorised for manufacture and use in Italy and Spain and could then be exported unchecked throughout the EU, it has been claimed.
Just months after it was licensed for use in Europe, two new studies showed a greater diversity of birds of prey, including some eagles, are more susceptible to its effects than previously thought.
Campaigners hope to reverse the decision in a bid to protect British and European bird numbers.
One paper published in the journal Bird Conservation International showed the results of tests carried out on two steppe eagles found dead at a cattle carcass dump in Rajasthan, India.
Both birds had diclofenac residue in their tissues and suffered kidney failure similar to Gyps vultures experimentally given diclofenac.
Conservationists argue steppe eagles are closely related to golden eagles and other globally vulnerable or declining Eurasian raptors.
They fear that all species in this genus, known as Aquila, are susceptible to diclofenac.
Dr Toby Galligan, RSPB conservation scientist, said: "We have known for some time that diclofenac is toxic to Gyps vultures, including the Eurasian griffon vulture, but we now know it is toxic to an Aquila eagle too.
"This suggests that the drug is fatal to a greater number of birds of prey in Asia, Europe and around the world.
"We had suspected as much from observed declines in non-Gyps vultures in Asia, but this study confirms our worst fears."
In another paper published in April's Bird Conservation International Dr Galligan led an examination of recent population trends in Egyptian and red-headed vultures in India.
That study shows population declines on a similar scale, providing indirect evidence that these species have been impacted by diclofenac as well. Mr Galligan said: "In light of recent developments in Europe, our findings take on an even more worrying meaning. All of Europe's charismatic Aquila eagles, like the Spanish imperial eagle and, closer to home, the golden eagle, are opportunistic scavengers and therefore could be at risk of diclofenac poisoning."
The UK's Veterinary Medicines Directorate (VMD) which regulates animal medicines, said that it is "taking the issue of diclofenac's risks to vulture populations seriously".
A spokesman added: "As a precautionary measure the VMD will not approve any requests from vets to import products containing diclofenac."