London's museums have increased security after raids by gangs stealing rhino horn.
Museums are responding to the problem by removing rhino horns from displays or by replacing them with replicas.
The Metropolitan Police has warned museums, auction houses and other institutions of the Europe-wide trend.
The thefts have been sparked by an increase in the value of rhino horn which is used in the traditional medicines of many Asian countries.
Police said they were aware of two break-ins in the UK - one in Surrey and one in Essex - but they warned that a number of gangs had visited museums on reconnaissance with a view to carrying out a theft.
Det Con Ian Lawson, of the Metropolitan Police's Art and Antiques Unit, said: "Some of the museums I know have taken the rhinos horns from their display, others have put a replica rhinos horn in place."
Thieves use a variety of methods, including "smash and grab" style raids and overnight burglaries. They have been known to use force when challenged.
Paolo Viscardi, deputy keeper of natural history at the Horniman Museum, said he was concerned about reports of tear gas being used by thieves and emphasised that staff safety was the museum's top priority.
"People who steal rhino horn can be quite aggressive. We don't want our staff to be open to those threats."
The south-east London museum has removed its rhino horn altogether, while the Natural History Museum in Kensington has put a replica in its place.
Earlier this month, Europol said it had uncovered an Irish organised crime group illegally trading rhino horn worth tens of thousands of euros as far afield as China.
The European police agency said it was working with Irish police and had drawn up an action plan to tackle the illegal trade.
Rhino poaching has also surged, with 200 of the endangered animals killed in South Africa last year.
The animal's horn is a prized ingredient in traditional Chinese medicine and is also used for decoration and to produce luxury goods.
Depending on the size and quality, a horn can be worth between 25,000 [£22,000] and 200,000 [£175,000] euros, according to Europol.