Holiday camp firm Butlin's says up to 34,000 guests at its resorts may have had their personal information stolen by hackers.
The company says the data in question included names, home addresses, email addresses and telephone numbers.
Managing director Dermot King apologised for the incident and said no financial information was compromised.
Butlin's has set up a "dedicated team" to contact guests who may have been affected.
"Butlin's take the security of our guest data very seriously and have improved a number of our security processes," said Mr King.
"I would like to apologise for any upset or inconvenience this incident might cause."
A spokeswoman for the Information Commissioner's Office said, "Butlins has made us aware of an incident and we will be making enquiries."
The BBC has contacted Butlin's for comment.
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