A recent survey of 500 marketing firms found that almost two-thirds admitted to experiencing a data breach involving the loss or theft of customer data over the past 24 months.
The information lost includes customer's names, addresses, email addresses, home phone numbers, gender and credit card details.
Paul Bates, managing director of Strongmail UK, said: "This is exactly the kind of information that criminals intent on committing identify fraud are after. The problem is that when data like this is lost or stolen we're often talking of thousands, or even millions, of private records at a time."
The words of warning come in conjunction with the launch of National Identity Fraud Prevention Week, which kicks off October 6 (today).
There have been a number of recent examples of third-party data loss in the UK including the details of 5,000 employees of the National Offender Management Service in England and Wales, including prison staff, lost by private firm EDS.
In August, production company Objective Productions lost data relating to 250 children, while working on a television program for the BBC.
Contractor PA Consulting was axed from a contract with the Home Office after the personal details of all 84,000 prisoners in England and Wales were lost on an unencrypted USB stick in September.
Bates said: "As UK businesses and government organisations continue to outsource confidential aspects of their business to third-party suppliers, the issue just gets bigger. Businesses have a moral obligation to keep private, personal customer data safe and secure. They should not be handing it out to third parties in a bid to cut costs."
This issue is compounded by the fact that in 90% of these cases, the loss or theft of customer data goes unreported since organisations feel that they are either not required to, or are unsure whether they have to, report the event.