Six lions have been put down at Longleat Safari Park sparking outrage among staff members.
The Daily Mail reports that an adult male lion called Henry, a lioness named Louise and four of her cubs were all put to death last month in an operation supervised by vets.
Bosses at the Wiltshire safari park, owned by the Marquess of Bath, have defended the decision, saying it was taken because of health risks after a population increase led to violent behaviour.
The decision has left some experts scratching their heads. Leading big cat expert John Knight said: "This could be part of a conservation programme. It is a debatable reason for putting down a lioness and her cubs but until all the details emerge it is difficult to understand their thinking."
The lions were reportedly injected with a lethal substance using a tranquiliser gun.
According to the Metro, a zoo source said: "I don't think there was a reason to do it. He (Henry) had been vasectomised so he couldn't breed. Henry could have been sold to another zoo.
"It all happened while the park was closed for the winter so no one knew what was going on. A lot of people who know what happened are very upset and angry."
A Longleat spokesman told the Daily Mail: "There has been a large increase in pregnancies among the lions, resulting in a 40 per cent increase in population. This has resulted in excessive violent behaviour, putting 21 of them at risk.
"Sadly one lion, Henry, had to be put down earlier this year due to injuries from an attack within the enclosure. The further lions referred to were put down due to associated and severe health risks.
"A further five lions from this enclosure will be moved to other premises. Longleat takes the utmost care in trying to protect the welfare and safety of all its animals."
Users on Twitter reacted to the Daily Mail's report, with @dolhpinseeker30 writing: "Use of both control at zoos should prevent this", and another, @AndrewKelly1962, saying: "We need a serious review of roles of zoos."
The news comes just a few days after pictures emerged of Longleat's lions happily playing in their enclosure, jumping out of trees.