The RSPCA has launched an investigation into the death of a lion at a controversial British zoo after it suffered from barbiturate poisoning thought to have been caused by contaminated meat.

Nero the African lion died at the South Lakes Safari Zoo in Cumbria in July last year, but the details of his death are only now coming to light thanks to a BBC2 documentary Trouble at the Zoo, which will air next week.

The zoo's chief executive Karen Brewer confirmed Nero had collapsed as a result of barbiturate poisoning and spent four days in intensive care before passing away, but the cause is yet to be established.

South Lakes Safari Zoo made headlines in 2013 when Sarah McClay, a 24-year-old animal keeper, was mauled by a tiger during feeding time, and later died as a result of her injuries.

Controversy further surrounded the zoo when it emerged almost 500 animals had died under its care between December 2013 and September 2016, including a jaguar who chewed off its own paw and two snow leopard cubs which were found partially eaten in their enclosure.

Other incidents included a tortoise being electrocuted, the decomposed body of a monkey being found behind a radiator, and lions and baboons being euthanised due to issues with space.

"We are deeply concerned to hear about new revelations relating to the sad death of Nero the lion at the zoo and as a result we will be investigating this further," a RSPCA spokesperson told The Independent.

"Despite new management and some reported improvements in welfare at the zoo last year, the RSPCA continues to have concerns about the conditions there.

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"Worryingly, a planned inspection of the zoo by Barrow Borough Council, which is responsible for issuing its licence, has been postponed.

"We continue to have concerns that the running of this zoo is having an impact on the welfare of the animals and recently wrote to the chief executive of the council urging him to carry out this inspection jointly with the RSPCA as a matter of urgency.

"We are very troubled about reports of further animal welfare concerns and we urge the programme makers to share any information they have."

The Independent has contacted South Lakes Safari Zoo for comment, but Ms Brewer told The Daily Mirror the zoo was "deeply saddened" by the death of Nero.

"Although we cannot determine 100 percent where the contamination came from it is essential to try to ensure it does not happen again," she added.

"Barbiturates can only be administered by a vet and could have come from Nero's food supply.

"We have an ongoing contract with a meat supplier to only deliver meat that has not received veterinary attention in any way.

"Nero was a firm favourite with both visitors and animal carers alike and he is deeply missed."

South Lakes Safari zoo to reduce number of animals in collection

John Woodcock, Labour MP for Barrow and Furness, told MailOnline: "These latest distressing revelations occurred under the nominally new management at the zoo and underline why the government must overhaul the whole inspection regime which is allowing this to happen.

"The suspicious circumstances around Nero’s alarming death surely merit a criminal investigation; if the police are not already involved I will be asking the documentary makers to submit any evidence they have."

The controversies surrounding South Lakes Safari Zoo led to its founder David Gill losing his license to continue running it.

Former staff subsequently formed the Cumbria Zoo Company Limited (CZCL), which has been operating the attraction since January 2017.

And despite concerns from the Captive Animals' Protection Society (Caps), which condemned the zoo as one of the worst it had ever seen, Barrow Borough Council administered a new license to CZCL in May 2017 to keep the zoo operational.

The BBC film crew behind Trouble at the Zoo were invited to shoot at South Lakes last year across a period of five months to document how CZCL was fighting to save the zoo's reputation by improving practices while keeping it financially afloat.