BT’s broadband services received the most complaints during the third quarter of 2016, show figures from regulator Ofcom.
It received 36 complaints for every 100,000 broadband customers – higher than the industry average of 20.
BT-owned companies, Plusnet and EE, came second and third in the broadband complaints table.
Vodafone had 18 complaints for every 100,000 of its mobile customers – three times the industry average of six.
As well as doing poorly on broadband, Plusnet and EE also received the most complaints about their fixed telephone services.
The rankings come soon after BT chairman Sir Michael Rake said it had more work to do to improve customer service.
“We apologise to customers we’ve let down,” said BT in a statement. “BT cares about the service it gives to customers and we know we need to do better.”
It added that some “exceptional” broadband outages during the third quarter contributed to the rise in complaints.
Bt said it was putting cash into service improvements and recruiting widely to bolster the number of staff who handle queries and complaints, The software it uses to handle complaints was also getting an overhaul, it added.
“Customers will see an improvement as these initiatives take effect,” it said.
The quarterly Ofcom study looks at the number of complaints lodged about landlines, broadband, mobiles and Pay TV services.
The complaints cover issues such as faults, billing and pricing as well as problems with services and how well they are set up by firms. They also include the ways telecoms firms deal with grievances.
Ofcom said it had seen an increase in complaints about broadband and landline services between July and September.
“We won’t stand for complacency when it comes to customer service,” said Lindsey Fussell, director of Ofcom’s consumer group in a statement. “We expect providers to make it a top priority and work hard to better serve their customers.”
Ofcom said it gathered data on complaints to help monitor how well companies were serving customers and to provide useful information for people who were looking to switch suppliers or sign up for new services.
Ms Fussell said it investigated when customers were let down and had the power to levy significant fines.
In October, Vodafone was hit with a £4.6m fine by Ofcom for breaking rules on handling complaints and misleading customers on what they received when they signed up for pay-as-you-go services.
Last year, a £1m fine was levied on EE for not doing enough to tell people their rights when they made a complaint.