WHSmith magazine subscribers have received emails with details of other customers in what the retailer describes as a “bug”.
Among the details that have been sent are names, addresses and phone numbers, BBC writes in a report.
The information has come from the retailer's "contact us" form, which has then been sent on to others, but WHSmith says no payment details was revealed.
"We have been alerted to a systems processing bug by I-subscribe, who manage our magazine subscriptions. It is a bug not a data breach," the retailer said.
"We believe that this has impacted fewer than 22 customers who left a message on the 'contact us' page where this bug was identified, that has resulted in some customers receiving emails that have been misdirected in error."
The number of people who received these emails is unknown, but BBC says it is being counted in thousands. WHSmith added that the problematic form had been taken down and those affected were being contacted with an apology.
"We can confirm that this issue has not impacted or compromised any customer passwords or payment details," it said.
The Information Commissioner's Office, which polices data security, said: "We are aware of an incident regarding WHSmith and are making enquiries."
WHSmith is a British retailer which operates a chain of high street, railway station, airport, port, hospital and motorway service station shops selling books, stationery, magazines, newspapers and entertainment products. Its headquarters are in Swindon, Wiltshire. Smith's is listed on the London Stock Exchange and is a constituent of the FTSE 250 Index. It was the first chain store company in the world, and was responsible for the creation of the ISBN book catalogue system.