The three lionesses and eight cubs are thought to be victims of villagers who blame wildlife for killing a cow.
The Uganda Wildlife Authority said it was launching an investigation after the pride was found dead at the Queen Elizabeth National Park.
It is believed only 19 lions are now left in the park, and experts warned of potential extinction across Africa.
There were immediate calls for “domestic animals” to be banned from the park and for the authority to compensate farmers whose cattle are killed by big cats in an effort to prevent future revenge attacks on wildlife.
Conservationists are worried the lion population in Africa is falling. A 2013 survey found the population had dropped by 30 per cent, and numbers in Uganda are now estimated at 400. The park is one of the last strongholds of the big cats in the country.
Wildlife experts also say the lions’ natural prey is declining, while the number of humans is rising and encroaching on lions’ traditional roaming areas, leading to greater conflict over land use.
The authority told The Independent: “Investigations will confirm the type of poison that was used. Investigations should lead to the identification, arrest and prosecution of the people behind this heinous act.”
But a spokesman denied natural prey was in decline. “The challenge is that there are people who graze cattle in the wildlife sanctuary so lions can easily prey on the cows. We have more than enough natural prey for the big cats,” he said.
Richard Kamara, a ranger with the authority, posted on Facebook: “The solution should be that all communities living within the park MUST not have any domestic animal, or else communities be relocated to other areas outside protected area.”
Mark Jones, associate director of the Born Free Foundation in the UK, said: “Uganda’s lions face an uncertain future, and cannot withstand these kinds of incidents.
"These animals need protecting from revenge killing and poaching through robust law enforcement, efforts to help local people live alongside wildlife without conflict, and ending the trade in their body parts.
"Without robust and swift action, the unimaginable disappearance of lions from Uganda and many other African countries they have called home for millennia, could become a cold reality.”
Bashir Hangi, communications manager for the Uganda Wildlife Authority (UWA), said a compensation project existed in that area but the authority hoped to have it enshrined in law.
“In the area where the lions were poisoned, there is a mechanism of compensating people; we are the people who can testify that they were compensated,” he said.
“We want a harmonious co-existence between wildlife conservation management and communities because we are all stakeholders in protecting wildlife.
“We are reviewing the UWA Act to put in place punitive measures for poachers, traffickers and all those found guilty of killing wildlife.
“Also we are proposing to include compensation for people in the Act. So it is still under review until when it is discussed by parliament and passed.”
The poisonings come just six weeks after the UN's World Wildlife Day focused on big cats and efforts to protect them.
The Uganda Carnivore Program, which works to save lions, leopards and hyenas, said: “Habitat loss due to human settlement and agriculture development, loss of prey population, and retaliatory killing by humans following livestock depredation are their main threats throughout Africa.”
Agricultural encroachment by people is happening along many of the park’s boundaries, the group said, creating conflict when crops and livestock take up areas formerly used by wildlife.
The Queen Elizabeth National Park, which covers more than 700 sq miles in Kasese district, is home to one of only two populations worldwide of tree-climbing lions. The other is in Tanzania.
The authority said in a statement: “The minister, tourism, wildlife and antiquities, Professor Ephraim Kamuntu, ED UWA, Sam Mwandha and other officials will address the media and public about the tragic death. This is therefore to invite all the wildlife and tourism fraternity to join in the ‘funeral’ of our big cats! *#sad_and_tragic_moments*”
The deaths happened near Hamukungu fishing village in the park, which is a popular tourist destination.
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