A tape containing the personal data of 51,000 UK customers has been lost by insurance company Zurich.
It confirmed that the back-up tape went missing during a routine transfer to a data storage centre in South Africa in August 2008.
Details of 550 people in South Africa and Botswana were also on the tape.
The company has written to all the customers whose details were on the tape, but there was no evidence the data had been misused, Zurich said.
The letters explain precautions available for general insurance customers whose details were exposed.
Zurich UK has brought in accountancy firm KPMG to conduct an investigation, it has stepped up security surrounding the transportation of tapes, and the UK Information Commissioner has been informed.
"We apologise to any customers affected by this unfortunate matter. We take the security of our customers' data very seriously. What has happened is unacceptable to us," said Annette Court, of Zurich.
"We are putting a great deal of investment into strengthening our internal processes to ensure that incidents of this nature do not happen again in the future."
The latest loss comes after a series of cases when personal details have been compromised.
A laptop computer with details of 109,000 members of six pension schemes run by the Pensions Trust was stolen in March.
In October 2008, a laptop containing personal details of more than 100,000 members of the Network Rail and British Transport Police pension schemes was stolen from the accountancy firm Deloitte.
The month before, the Ministry of Defence lost data about 100,000 of its personnel.
The most high-profile loss of personal information came in November 2007, when HM Revenue and Customs (HMRC) lost two computer discs.
They held the entire child benefit database, including the personal details of 25 million people, covering 7.25 million families.