Butlin's has confirmed that up to 34,000 guest records may have been hacked by an unauthorised third party.
The data accessed included names, email addresses, telephone numbers and postal addresses.
The chain of holiday camps has responded to the incident, ensuring guests that their payment details remain secure and were not compromised.
An investigation has found that no fraudulent activity appears to have taken place in this breach.
Affected guests are being contacted by the Butlin's team.