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 is investigating the breach, which prompted notifications to shoppers claiming hackers had "fully compromised" its data. Asos said basic personal information including name and contact details may have been accessed, according to The Guardian.
The retailer stated it does not believe that payment card records or passwords were compromised in the incident.
Asos has not provided additional detail on the timeline of the breach, when the unauthorised access was discovered, or how many customers may be affected.