Fortnum and Mason has said around 23,000 people have been contacted over a data breach.
The upmarket food retailer, which has a history stretching back more than 300 years and is known for its hampers and its links to royalty, said for the majority of people, only their email address has been exposed.
It added that for a smaller proportion of customers, other data such as addresses, contact numbers and social media handles had been included.
Fortnum and Mason said in a statement that on Friday last week, Typeform, a company that provides services it has used in the past to collect survey responses, notified the store "that they had suffered a data breach and unfortunately some of our data had been compromised".
The statement said: "The data of approximately 23,000 competition and survey participants who inputted into a Typeform form has been involved in this breach."
Fortnum and Mason said: "No-one's bank details or passwords have been involved, and money and accounts are safe.
"All those affected have been contacted."
It continued: "There has been no breach of Fortnum and Mason's website or database, and all data which we hold is unaffected by this breach."
It said Typeform forms on the store's website have been disabled.
The statement continued: "We have been informed that Typeform have fixed the root cause and are undertaking forensic investigations."