Seven-year-old lion Mojo was involved in a fight during the night with the lionesses at Knowsley Safari Park and was fatally wounded.
It was during checks on Tuesday keepers discovered Mojo's injuries.
A vet team at the safari park, near Liverpool, worked quickly to help him with pain relief but he died while recovering from the anaesthetic.
The post-mortem revealed Mojo had muscular injuries which were not immediately visible.
These were likely to cause high levels of toxicity in his system meaning there was no chance of recovery.
Keepers had noticed some hostility from the other lions towards Mojo.
Separating him from the pride would have meant spending the rest of his life alone.
Then integrating him with another pride would likely have resulted in the same fate.
The lion's brother, Scooter, was not harmed in the incident.
He has a strong position in the pride and a similar occurrence is unlikely.
He remains the only male in the pride.
This latest incident comes after two similar occurrences earlier this month, with a female tiger savaged to death by her new mate at London Zoo and 13-year-old female tiger Shouri killed by two others at Longleat Safari Park.
Attacks on pride males happen regularly in the wild because male lions need to be accepted into the pride and as a result conflict is a normal part of lion social behaviour.