According to Credit.com, which cited two client letters as a source, the details were contained on two CD-ROMs which were password protected but not encrypted.
The discs were lost after being posted to the New York State Department of Taxation and Finance - it is thought the package containing them reached the correct address but not the desk of the intended recipient.
Jim Wiggins, spokesman for the organisation, told the news provider: âThereâs no evidence that there was any criminal intent here, or actual misuse of this information.â
However, he added: âWeâre going to work with the state to see if we can improve the security of this data transmission.â
Names, addresses, account and tax identification numbers are believed to be among the lost details.
Morgan Stanley Barney Smith was informed about the loss on 8 June while clients were informed on 24 June following a search for the package.
The news of the data loss follows the recent hack attack on Citigroupâs North American credit card unit.
An estimated 360,000 accounts were targeted with Citi losing $2.7 million.
By Jim Ottewill