The regulator found that between July 2011 and April last year, the UK’s largest mobile phone operator had misled customers by failing to inform them of their right to an independent review.

If consumers are unhappy with the way their operator is dealing with a complaint within eight weeks of it being made, they can write a “deadlock letter”, giving notice that they intend to seek an independent judgment.

EE failed to inform customers that they could request a deadlock letter, and those who asked for one either did not receive it, or were told by EE that the operator did not issue them.

Ofcom’s investigation of EE formed part of a wider review of slack complaint handling in the mobile sector. Competitor Three was fined £250,000 for its negligent handling of customer complaints last year.

“It’s vital that customers can access all the information they need when they’re pursuing a complaint,” said Claudio Pollack, consumer and content group director at Ofcom.

“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.”