Ofcom has fined EE 1 million for failing to give customers adequate or accurate information regarding its customer complaints process.
The fine is one of the biggest Ofcom has issued over poor customer service.
The regulator found that between July 2011 and April 2014, the mobile firm misled customers making complaints, by failing to send out written notifications to that referenced customers' rights to take their complaint to an alternative dispute resolution (ADR) eight weeks after they first raised their complaint.
Ofcom added that EE also "failed to state in its Customer Complaints Code that, where relevant, customers could access its ADR scheme by requesting a deadlock letter'".
"A number of customers who had requested a deadlock letter' during this time were not sent them as required, and in some cases customers were told by EE that letters of this type were not issued," Ofcom said in a statement.
The firm also neglected to tell customers that they could use its ADR scheme for free.
Claudio Pollack, Ofcom's consumer and content group director, said: "It's vital that customers can access all the information they need when they're pursuing a complaint.
"Ofcom imposes strict rules on how providers must handle complaints and treats any breach of these rules very seriously. The fine imposed against EE takes account of the serious failings that occurred in the company's complaints handling, and the extended period over which these took place."
In a statement, EE said that while it made no excuses, it has "identified issues" in its complaints handling in 2013. "We have made considerable improvements since then," it said.
"Ofcom's current figures highlight that complaints into Ofcom about EE have fallen by 50% in the past year alone and, while even one complaint is one too many, we're working tirelessly not only to improve the handling of complaints but also to identify root causes, and fix problems customers have with us, to ultimately achieve our goal of offering the best service in the market.," the mobile operator added.
Richard Neudegg, regulations expert at uSwitch.com, said that the size of the fine showed how seriously Ofcom was taking failure to follow rules on complaints.
"If telecoms companies don't resolve consumers' issues internally, people have a right to contact one of two free-to-use resolution schemes to get things sorted," he said.
"Access to an Ombudsman is of huge importance. It gives people who feel they've been let down by a service provider another route to resolution."
"Part of the problem is the telecoms market's complaints procedure is more confusing than other sectors. There isn't one single Ombudsman, there are two that companies may direct customers to. So it's extremely important information on how to access these is completely clear."
BT is in the process of buying EE for 12.5 billion, but the merging of the firms may create the worst customer service department in telecoms, according to an Ofcom customer satisfaction survey released in May.
EE was the mobile network with the most complaints for two of the past three quarters in 2014, while BT suffered the most complaints as a TV provider.
