Wildlife parks in south-east England are on high-alert after a tip off about a plot to hunt rhinos.
Poachers in Africa regularly shoot rhinos to make off with their horns, which are highly valued for alternative medicine in parts of Asia.
Two animal parks in Kent - Port Lympne and Howletts - have called for volunteers to help them carry out 24-hour patrols, while police have also stepped up surveillance.
Pc Michael Laidlow, wildlife crime officer for Kent Police, said: "There was anonymous information through Crimestoppers saying there was a possible attack on rhinos.
"We've spoken to all the owners of rhinos in the South East and they are taking measures to combat it.
"All our wildlife officers are aware of the information and we're taking steps to increase patrols in the area."
Rhino horn is worth £65,000 a kilo, which makes it more valuable than cocaine, heroin and gold.
Two men were jailed after a failed bid to steal a rhino head from a museum in Norwich in February last year.
They were part of a gang who were stopped by museum staff and visitors after smashing a glass case at Castle Museum.
Bob O'Connor, managing director of Howletts and Port Lympne, said: "What we're worried about is criminals taking it to the next level.
"We've upped our night-time patrols, we doubled that straight away with our keepers who live on site.
"We have to take this as a credible threat."
There are 20 black rhinos at the parks, out of just 45 in the whole of Europe.
"You can get up to £200,000 for a rhino horn," Mr O'Connor said. "It's used in Far Eastern medicine. It's claimed to have medicinal qualities, which is a nonsense."
Earlier this month, the Duke of Cambridge urged a global conference on wildlife protection to do more to tackle the illegal killing of rhinos. He visited Port Lympne last year to meet and feed rhinos.