The personal details of hundreds of thousands of African customers also went missing.
A computer tape containing the customer information went missing over a year ago when it was being transferred to a storage centre in South Africa, the company disclosed.
Zurich Insurance, which called in KPMG to investigate, said there is no evidence that customer details had been used to commit fraud.
Annette Court, CEO Europe general insurance of Zurich Financial Services Group, said the company is investing money to improve its data security.
"Protecting our customers' interest is at the top of our agenda. 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."
But Jamie Cowper, marketing director EMEA at data encryption firm PGP Corporation, said there are often dangers when customer data is held overseas.
"Zurich UK's customers might be surprised to hear that their data is being kept in South Africa, a country which is yet to pass the Protection of Personal Information Bill, its equivalent of the Data Protection Act," he added. " However, global trends around data outsourcing mean that confidential customer data could be held absolutely anywhere."