BT clocked up 34 complaints per 100,000 subscribers in the first three months of 2017, up slightly on the previous quarter and higher than a year ago. It was followed by TalkTalk, then BT-owned Plusnet and EE.
Meanwhile, its rivals Virgin Media and Sky enjoyed much better numbers – just 13 and eight complaints per 100,000, respectively.
BT was also the most complained about pay-TV brand, for the seventh successive quarter, with 19 complaints per 100,000. This was almost ten times higher than Sky, with two, and four times higher than the industry average, on five.
Vodafone was once again the most complained about mobile network, by some distance, although it has managed to bring its complaints down to the lowest level in two years, with 17 per 100,000. This is still more than four times higher than EE (four), O2 (four) and Three (three).
Lindsey Fussell, Ofcom's consumer group director, said: "Our report shows that, for a number of providers, there is still much room for improvement.
"Phone and broadband companies must take all possible steps to improve their complaints handling and customer service, and we expect this to be their number one priority."