The survey of 900 marketers was commissioned by in-house email marketing software specialist Strongmail and carried out by the Ponemon Institute.
The results showed that 61% of UK marketers said they had experienced a data breach involving the loss or theft of consumer information over the past 24 months.
In 90% of those cases the loss or theft went unreported because they felt that they were either not required to, or were unsure whether they had to, report the incident to the affected customer.
Outsourcing appears to make data breaches more likely to happen. Data breaches were 42% more likely to be attributed directly to third parties than to in-house personnel.
Narrowing down the sample to companies that outsource their email marketing, 78% of marketers said they had experienced a data breach in the past 24 months, compared with 61% overall.
The practice of outsourcing marketing was criticised by Strongmail's UK managing director Paul Bates.
He said: "The fact is confidential customer data doesn't travel well and providing it to third parties for outbound marketing purposes can, as the research shows, be a risky proposition."