NEW YORK (REUTERS) - Madonna attempted Tuesday (July 18) to stop an upcoming auction of a prison break-up letter from rapper Tupac Shakur - and a note in which she called singer Whitney Houston and actress Sharon Stone "horribly mediocre".

The singer filed a request in New York Supreme Court for a temporary restraining order against New York auction house Gotta Have It Collectibles to halt the sale, due to start Wednesday.

Madonna said in court documents that she was not aware until reading press reports that many of the items listed for auction were no longer in her possession.

The items were consigned to auction by Ms Darlene Lutz, whom Madonna described in court documents as a former friend

The singer said she had "betrayed my trust in an outrageous effort to obtain my possessions without my knowledge or consent".

A representative for the auction house and Ms Lutz said Madonna's effort to stop the sale was "a completely baseless and meritless action", and they would challenge her claims in court.