Two former zoo keepers will face no further action over claims they mistreated elephants at the attraction.
Three workers at Twycross Zoo were dismissed and then arrested after the zoo reported its concerns to Leicestershire Police last year.
The force handed a file to the Crown Prosecution Service relating to two of the men after it was decided no further action would be taken against a third.
The CPS said today: "This has been examined in detail and we have informed Leicestershire Police that there is insufficient evidence for a realistic prospect of conviction, in that the conduct shown is not sufficient to prove a criminal offence under the Animal Welfare Act 2006."
Staff at Twycross Zoo have revealed how they tempted a group of straying chimpanzees back into their correct living areas:
Twycross Zoo was closed to the public after "some chimpanzees" got into an area where they should not have been.
A spokeswoman for Leicestershire Police said: "A call came in at 9.35am from the zoo to let us know that some chimpanzees, that were in the process of being moved from one building to another.
"In the process of this they have perhaps gone into an area of that building that they shouldn't have been in.
"They don't appear to have actually physically escaped. As a precaution, we've been called by the zoo."
The spokeswoman said they stopped people going into the park for a "short time", and added that there was no danger to anyone in the park and no animals were on the loose.
Twycross Zoo has re-opened following an 'animal incident'. It has not be confirmed what animal was involved.
A spokesperson for Twycross Zoo say they are dealing with an 'animal incident' on site and the zoo is currently closed to the public:
Twycross Zoo have announced that the gorilla born in early January is a boy.
Mum, Ozala, gave birth on 3rd January 2013. Ozala, was, at first, very protective of her newborn and so staff were unable to get close to the infant.
“Ozala was so protective of her newborn infant that she held it very close and it is only now that the infant is bigger, and Ozala has become more relaxed, that we have been able to see the infant closely enough to determine its gender."
Charlotte Macdonald, Curator of Living Collections
The western lowland gorilla is a critically endangered species.