The CDs reportedly contained information such as clients’ names, addresses, account and tax identification number, income earned on investments in 2010, and some Social Security numbers.

They were lost once they had been mailed by the company to the New York State Department of Taxation and Finance. Although they are password protected, they are not encrypted.

The parcel arrived intact at the correct department, but the CDs were missing by the time it reached the intended recipient, Jim Wiggins, a spokesperson for the broker, is quoted as saying.

"We searched every facility the package passed through," Wiggins reportedly said.

He added that so far MSSB had not seen any evidence of criminal intent or misuse of the information, and that the company is offering a year of credit-monitoring services from a third party for clients who had their Social Security numbers compromised.

The data leak comes at a time of heightened concern about privacy, with Citigroup having been the target of a cyber attack only last month. In that case, a massive computer hacking attack resulted in hundreds of thousands of bank card holders having their details compromised.