The lion has been deteriorating for the last 25 days and has stopped eating and drinking water. It has been undergoing treatment for diarrhea.
"Shyamal did respond to treatment in between. However, his condition is worsening because he is not responding to medicines well, "said Patel.
Shyamal is the only lion in Surat's zoo.
Asiatic lions, different from African lions, with a characteristic skin fold on their bellies and thinner manes on the males , once roamed most of Asia.
India is also struggling to save its endangered tigers, as people invade their habitat and poachers kill them for body parts that fetch huge sums in the international black market.