UPDATE: BBC Earth has today (11 December) learnt that a total of three lions may have died. Another of the young male lions, Alan, did not make it. Vets had to euthanise him to prevent further suffering. There is no news on the whereabouts of the female lioness Sienna, but it is assumed a lion body that has been discovered is her. These two deaths follow that of another lion, reported below on 7 December 2015.
ORIGINAL ARTICLE: One of the world's most renowned lion prides has been poisoned, resulting in at least one big cat being killed.
It is unclear who targeted the cats, but lions have previously been poisoned in a bid to prevent them attacking livestock kept by local farmers.
The lions were members of the Marsh pride, a family of cats living in Kenya that have been filmed for many years by the BBC.
One of the pride's female lions, known as Bibi, was discovered dead after the poisoning.
Pictured above, she was made famous from her appearances on the BBC's Big Cat Diary from 1996 to 2008.
Bibi was discovered dead on Monday 7 December in the Masai Mara reserve in Kenya. A BBC wildlife crew was at the scene and reported that she was found "lying on her side, foaming at the mouth, fitting and panting".
"She was not in the main pride on 6 December and we believe nobody had located her until she was found near death," said a crew member.
At least six others in her group were also poisoned, reports suggest.
The David Sheldrick Wildlife Trust was quickly alerted that several lions were "acting strangely, collapsing and suffering from spasms". The trust's vets are now at the scene, treating the other poisoned members of the group.
One lioness, Sienna, remains unaccounted for, the charity reports. Her two-year-old cub was also affected by the incident and is responding well to treatment.
BBC wildlife cameraman Mark MacEwan was at the scene. He tweeted: "One of the more unpleasant sights of my career this morning, last night the lions of the marsh pride in the Mara were poisoned." He also posted a picture of Bibi the lion, an image that some people may find distressing.
BBC Earth cannot confirm who was behind the latest attack.
However, the Kenyan Wildlife Services has made three arrests. If found guilty, any perpetrators face hefty fines or life imprisonment.
This is not the first time such an incident has occurred. Local groups have done so previously by leaving out poisoned meat. This is often done in retaliation to lions attacking livestock, as many cattle encroach on the reserve each night.
"Five more lions are in critical condition. The poisoning of lions in the Mara is a disgrace to all Kenyans and is a very serious offence," said Paula Kahumbu, of Kenya's Wildlife Direct charity.
Earlier this year, another reknowned lion, known as Cecil, was killed by a trophy hunter in Zimbabwe, sparking an international outcry on social media.
Read BBC Earth's "Seven reasons to love lions".