EE has been fined £1m by the telecoms regulator for "serious" failures in its handling of customer complaints.

OFCOM said its investigation, over almost three years from July 2011, found EE "did not provide certain customers with accurate or adequate information about their right to take their complaint to an alternative dispute resolution (ADR) scheme.

It said the company failed to send out written notifications to a number of customers and did not state in its customer complaints code that, where relevant, customers could access its ADR scheme by requesting a so-called ‘deadlock letter’.

In some cases, OFCOM added, customers were told by EE that letters of this type were not issued.

The regulator also found that EE sent paper bills to Orange customers and written notifications to Orange, 4GEE and T-Mobile customers that did not reference that they can use its ADR scheme for free.

After issuing the penalty, OFCOM said the firm, which is on course to be bought by BT, had since amended its complaints code.

The money is payable within 20 days and goes to the Treasury.

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."

An EE spokeswoman said: "This fine relates to our historic performance regarding complaints handling, collected from 2011 to April 2014.

"While this in no way excuses it, it is important to note that we identified issues in our complaints handling and began our programme to tackle these problems head on in 2013, before OFCOM started their investigation.

"We have made considerable improvements since then."