Citigroup has lost credit information on 3.9 million of its customers, the company has warned.

The data, which contains customer names, Social Security numbers and payment history information, was stored on computer tapes being sent to a credit bureau, the bank said. It is now mailing a letter to affected customers, advising them of the situation and offering a number of tips on how they can prevent identity theft.

One such tip is: "Do not send sensitive information unless it is encrypted on a secure Web site", and the bank appears to be taking that piece of advice to heart. From next month, it will begin sending credit bureau data "electronically in encrypted form," the company said.

The lost such a huge amount of data comes just a week after a number of US bank, including Wachovia, Bank of America, Commerce Bancorp and PNC Bank, admitted that 676,000 customer records had been compromised.

In addition to data on US CitiFinancial Branch Network customers, the tapes contain information on closed CitiFinancial Retail Services accounts.

Because of the sensitivity of the missing information, Citigroup and UPS - the courier company used for the tapes - have informed "a number of law enforcement agencies and regulatory authorities," said a UPS spokesman.

UPS is still trying to understand how the package went missing. "We're delivering on average 14.1 million packages every day and the technology that we have today is such that it is very unusual for us to find ourselves in a situation where we literally can't find a package," the spokesman said. "We have to determine what did go wrong so we can fix it."

It is not the first time tapes containing large quantities of personal information have gone missing in transit. In March, Time Warner blamed data storage company Iron Mountain for the disappearance of a shipment containing details on about 600,000 current and former employees.