Telecoms provider Vodafone is facing the prospect of a large fine, after the regulator found that it failed to deal with customer complaints properly.
Ofcom said it believed that Vodafone had broken the rules on handling complaints over a two-year period up to November 2015.
It accused Vodafone of failing to have the proper procedures in place.
Vodafone - which can be fined up to 10% of its turnover - said it would study the report in detail before responding.
Ofcom also accused the company of failing to send out written notifications to customers where a complaint was not resolved after eight weeks - a requirement under the industry code of practice.
Figures from the regulator show that Vodafone is the most complained-about network in the UK.
In March this year, the number of customers complaining about Vodafone rose from 20 to 32 per 100,000.
In a statement, a Vodafone spokesperson said, "We note Ofcom's statement on this investigation, with which we have been cooperating fully. We will be reviewing the Ofcom report in detail before deciding what representations to make."
Ofcom launched its inquiry into Vodafone last summer.
The company now has a chance to respond before Ofcom announces its final decision.