Mobile operator Three has admitted that some customers were able to view the mobile account details of other Three users via their accounts yesterday.

The incident emerged as customers complained to Three via social media.

One customer Mark Thompson, wrote on Three's Facebook page: “Care to explain just how my details have been shared, how many people have had access to my personal information, for how long, and how many of your other customers have had their details leaked by yourselves to other members of the public as well?”

A Three spokesman told The Guardian the firm is investigating the incident. “We are aware of a small number of customers who may have been able to view the mobile account details of other Three users using My3.

“No financial details were viewable during this time and we are investigating the matter.”

The Information Commissioner’s Office said it “will be looking into this potential incident involving Three”.

A spokeswoman for the regulator said: “Data protection law requires organisations to keep any personal information they hold secure. It’s our job to act on behalf of consumers to see whether that’s happened and take appropriate action if it has not.”

Last year Three admitted its customer database was compromised by hackers and more than 130,000 customers had had their account data exposed.

Three is owned by Hong Kong-based company CK Hutchison and has 9 million users in the UK. The firm had hoped to merge with Telefonica's O2, but last year the £10.5bn deal was smacked down by European regulators. ®