Staff at Longleat Safari and Adventure Park are said to be "outraged" after six lions were put down.
An adult male called Henry, a lioness named Louisa and four of her cubs were all put to death last month in an operation supervised by vets.
The safari park confirmed it had taken the decision on its Facebook page following an article in the Mail on Sunday, which reported that staff were “outraged” by the move.
Bosses insist the decision was taken because of health risks after a population increase led to violent behaviour, adding that it takes “the utmost care in trying to protect the welfare and safety of all our animals”.
A full statement released by the park yesterday evening said: “The lioness Louisa arrived at Longleat in 2011 as an 18-month-old cub.
“At the age of 13 months, at the collection where she was previously held, Louisa exhibited neurological clinical signs which were thought to have been caused by inadequate nutrition leading to hypovitaminosis A.
“This was treated at the time but never fully resolved itself and she continued to exhibit clinical signs of head tilt and tremors throughout her life.
“Despite suitable nutrition these neurological signs were present in her cubs, which were clearly distinct from other litters in the pride as they all individually exhibited adverse neurological signs such as ataxia, incoordination and odd aggressive behaviour that were not considered normal or appropriate compared to other animals within the collection.
“Reviewing the genetic lineage of Louisa and her cubs it was found both Louisa's parents exhibited relatively high levels of inbreeding, prior to arrival, at a grand parentage level and great-grand parentage level (in some cases grandparents and great grandparents being the same animals).
“Further reviews of the pathology of related animals revealed a high level of brain tumours, which had not previously been reported in lions, as well as a general failure of normal neurological development.
“Longleat has never seen these problems in the many other cubs born here over the years and has an extremely good nutritional programme meaning that dietary inadequacies have never been an issue.
“The only consistent link with all these neurological developmental disorders has been Louisa and this was attributed to her confused and poorly managed genetic history prior to her arrival at Longleat.
“Longleat believes it would not have been responsible to translocate these animals to another collection, nor would any responsible zoological collection accept this particular group of lions, with the known high associated risks of neurological disorders and other genetically related health issues being passed on to later generations.
“After considering the pressures in the group, due to the recent increase in pregnancies, and the developmental disorders present in the cubs it was reluctantly decided that euthanasia was the responsible option for these individuals.
“Henry was a separate case, and his injuries were a result of aggression from both his brother and Louisa, who attacked him on the January 7.
“His wounds were severe, and despite veterinary review and management, it was decided euthanasia was the only humane option on welfare grounds.
“These decisions involve communication with all of our current staff, management team and with independent external ethical reviews undertaken to ensure we are consistent with best practice.”