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The alarm was activated at about 8.15pm on February 21, the night before the auctioneers' Country House Sale. Police arrived just ten minutes later and found that the external doors had been forced and identified vehicle tracks across a field.

The moth-eaten head, with its 19in (49cm) anterior and posterior horns visibly attached by staples, was one of eight African black rhino trophies included in the February 22 sale.

While the others – all shield-mounted horns – were kept overnight in the strong room, the 70kg head with an estimate of £20,000-30,000 had been secured to plates on a wall and remained in the saleroom. The thieves, who appeared to be aware of the short period of time between the saleroom closing and the arrival of overnight security staff, successfully levered it off the wall by shearing six screws.

PC Andy Long, wildlife and environmental crime officer at Essex Police, said: "This seems to be a targeted burglary. It is a valuable item in its own right, but if sold into the traditional medicine market would have a much greater value."

Police, who are taking the incident seriously given the nature of the item stolen, believe another break-in at Sworders earlier in February – shortly after the large cache of rhinoceros horns were first advertised for sale – could be related.

Under the new CITES guidelines published last week, DEFRA no longer recognise mounted rhino horn trophies as 'worked' items and they are illegal to sell. However, contrary to last week's report, full taxidermied heads with horn(s) attached, such as this one, will be considered to meet the worked definition and can be sold.

Permission does not cover sections of head with horn(s) attached (e.g. such as a pair of horns joined by a section of skin/skull), which will not be considered to meet the worked definition.

The sale of specimens advertised for auction prior to the Friday, February 18 announcement has been permitted. The other seven African black rhino trophies in Sworders' sale took a total of £368,000 plus premium.

Anyone with information is asked to contact police on 0300 333 4444 quoting the incident number EP/20110221/1134.