The Government insisted the location of dozens of Oxfordshire fields used to grow the raw materials for heroin must stay secret - to stop people stealing it.

The Home Office rejected a Freedom of Information (FOI) request lodged by the Oxford Mail to find out the locations of 10 sites in the county used to cultivate 281 hectares of opium poppies last year for medicinal use.

The poppies, from which the illegal Class A drug heroin is derived, are grown to produce legal morphine, used by the NHS to relieve pain.

In February, after revealing the existence of the 10 secret sites for the first time, the Oxford Mail asked for a detailed list of locations. The Home Office admitted it held the information, but said it was "not obliged" to disclose it.

A Home Office official said disclosure "would be likely to prejudice the prevention of crime".

He said: "In view of the nature of the crop being cultivated, disclosing precise details of the growing sites could encourage the commission of crime through trespass and pilferage of the crop."

Oxfordshire's tally of 10 opium sites, first revealed in a Parliamentary written answer published by Home Office Minister Vernon Coaker, was second only to that of Hampshire, where 1,238 hectares were grown across 26 locations in 2007.