Two brown bears have been shot dead at Whipsnade Zoo after escaping their enclosure.
The female animals, named Snow White and Sleeping Beauty, were able to climb out after a tree blew over in strong winds - creating a "bridge" over the wall of their area.
They attacked a wild boar in a neighbouring section, and zoo bosses said staff had no choice but to euthanise the animals.
The tragedy unfolded shortly before 8am today, when the attraction was closed to the public.
Chief curator Malcolm Fitzpatrick said it was an "incredibly upsetting and difficult day".
In an email to staff he said: "As brown bears are strong and dangerous predators, our first priority is safety – we must quickly make decisions informed by our experience and expertise to protect our people, guests and our other animals."
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He continued: "No one wants to be the one to make that call, but when there’s an immediate threat to human life, the decision is made for you."
Mr Fitzpatrick said tranquilising the bears was not an option, as this takes 20 minutes to work and animals can become "unpredictable and aggressive as adrenaline is coursing through them".
"That’s a risk we couldn’t take," he said.
"We had to intervene immediately to ensure the bears didn’t get out of the boar enclosure, which has a low fence."
He said that the fallen tree had formed a "bridge" across a public footpath into the wild boar enclosure.
Thankfully one bear in the enclosure, named Cinderella, did not escape from the enclosure and was unharmed. She was called to an indoor den and secured.
The fallen tree was then removed and the injured boar was given medical attention.
The Zoological Society of London (ZSL), which runs the Bedfordshire zoo, said a full investigation will be carried out to prevent any similar incidents in the future.
Mr Fitzpatrick added: "This situation is something we train to deal with through regular, rigorous drills - but one that we always hope we'll never have to face.
"I'm devastated by the outcome of this morning's incident, but I'm confident that our actions prevented any further loss of life."
His email continued: "We will of course be conducting a full investigation into this incident, so we can do everything in our power to ensure nothing like this happens again.
"For now, my priority is looking after the people involved, and the welfare of our affected animals.
"You will know the close relationship our keepers have to the animals they look after, and therefore the impact of today’s events on them.
"I know that all colleagues at Whipsnade and across ZSL will extend their sympathies to the team for this shared loss."
