A safari park has been criticised by government vets for keeping lions in a "very crowded" overnight pen for 18 hours a day over the winter.
Woburn Safari Park, which is building a new lion house due to open this summer, was inspected by officials from Defra in January following complaints.
A report says the "inadequate" building was "structurally unsound" but praises the Bedfordshire park for other pens.
Woburn Safari Park said lions were free to move within 10 linked pens.
The inspection was carried out by Defra and officers from Central Bedfordshire Council, the zoo's licensing authority, following concerns from a former worker.
It was followed up by a second visit in February and a further inspection earlier this month.
Reports on the latest findings, on 10 and 11 June, are yet to be published.
Sea lion discomfort
The park was also issued with a prohibition notice by the council in April to improve fences enclosing bull elephant Raja because of a "real and present likelihood of escape".
The notice was withdrawn the following day when the council acknowledged its conditions were being met, the park said.
It also said it had removed its sea lions to a facility with sea water, after an internal report in May last year discovered the chlorinated water in its own enclosure was causing eye discomfort.
The report on lions, which was compiled in January, said: "There were clear signs of fighting between the animals; the overnight house was inadequate in space provision and facilities for the animals, structurally unsound and unsafe to operate."
It found that staff going near the enclosure were armed with a shotgun in case the building deteriorated and the animals escaped.
"The lions were confined in the winter season for unreasonable lengths of time," it added.
"However, it was clear that the Woburn management were acting to improve the situation."
The report also stated that safari park staff should acknowledge problems of fighting between the lions and discuss ways to improve harmony, which may include moving animals or even euthanasia.
The inspectors concluded that the new enclosure did "not come close" to meeting the standards of the World Association of Zoos and Aquariums (WAZA) in terms of space, but it would give lions access to an outside run.
"The inspectors were greatly impressed by several other new animal houses at Woburn and would hope that the new lion house will be as successful," the report concluded.
"It is for this reason that we advise the management to accept this report and review the design and management of the new accommodation accordingly."
A spokesman for Woburn Safari Park said: "The new overnight facility will be bigger than most zoo facilities for lions and the existing day-time facility is bigger than most UK zoos in their entirety."
He added that Defra inspectors had "misunderstood" that the 10 pens within the existing lion house were completely interconnectable, allowing animals to move between them.
"Housing animals in buildings overnight is routine practice in all zoos for essential management, security and welfare reasons," he said.
Craig Redmond, campaigns director at the Captive Animals' Protection Society (CAPS) said the evidence it had seen was "the most shocking scenario at a UK zoo".
"We are calling for the local council, Defra and zoo trade bodies to carry out an urgent enquiry into how these conditions were allowed to continue for a long period of time, without action being taken earlier or zoo inspectors made aware."
The council said it was awaiting detailed reports from the vets before licensing conditions are updated.