Asos shares fell nearly 10 percent after the online fashion retailer disclosed unauthorised access to its app systems by an unidentified third party.
The company said it is investigating the breach. Shoppers received notifications claiming hackers had "fully compromised" its data.
Asos stated that basic personal information including names and contact details may have been accessed. The retailer said it does not believe payment card records or passwords were compromised in the incident.
The company has not provided details on the scope of affected customer accounts, the timeline of the breach, or when the unauthorised access was first detected.