* Law equating cannabis with heroin declared "illegitimate"
* Some 10,000 prisoners may be released
* Law's author says sends "devastating" message to youth
ROME, Feb 12 (Reuters) - Italy's constitutional court on Wednesday struck down a drug law that tripled sentences for selling, cultivating or possessing cannabis and which has been blamed for causing prison overcrowding.
The constitutional court said the law, which was passed in 2006 by Silvio Berlusconi's conservative government, was "illegitimate", without giving further details. Some estimates suggest 10,000 people may be released from jail as a result.
The law was the primary cause of Italy's prison overcrowding problem, according to prison rights group Antigone, which said that 40 percent of all inmates were serving sentences for drug crimes.
Italian jails are the most crowded in the European Union, with around 62,000 detainees held in cells built for fewer than 48,000, according to official data.
The law classified marijuana and hashish as equal, in legal terms, to cocaine and heroin, raising sentences for cultivation, sale and trafficking to 6-20 years from 2-6 years previously.
After the court's ruling, the drug law previously in place will automatically take effect, under which crimes related to "hard" drugs like cocaine and heroin carry lengthier sentences than cannabis.
Neither law made it a criminal offence to consume cannabis but both outlawed its possession.
"The so-called drug war as conceived in North America has been lost and it's time to return to rational rules that distinguish between substances," Franco Corleone, of the human rights group Society of Reason, told Reuters.
Senator Carlo Giovanardi, one of the original architects of the stricter law, said the ruling was a "devastating choice from a scientific viewpoint and in the message it sends to young people that some drugs are less dangerous than others". (Reporting by Steve Scherer; Editing by Robin Pomeroy)