EE could face an Ofcom fine after the media regulator found there are "reasonable grounds" to believe the mobile broadband provider broke its rules.
Following a year-long investigation, Ofcom thinks it is likely that EE contravened GC14.4 of its General Conditions, which relates to customer complaints handling.
GC14.4 requires operators to provide consumers and small business customers an "effective, transparent and accessible means for tackling complaints about their service".
Ofcom suspects this condition was breached at EE, and as such, has issued a notification to EE under section 96A of the Communications Act 2003.
"Specifically, Ofcom has reasonable grounds for believing that over the period from July 22nd 2011 to April 8th 2014, EE contravened GC14.4 by failing to have and comply with procedures that conform to the Ofcom Approved Code of Practice for Complaints Handling," the regulator stated.
This applies to complaints made by home and small business customers about EE's public electronic communications services, Ofcom added.
EE has been asked to respond to the findings, and the case continues.