TalkTalk has failed to improve its standing on customer complaints in Ofcom's annual survey - the broadband provider has come bottom of the table for the third year running.
Its customers were most likely to be unhappy with the outcome of complaints.
A spokeswoman for TalkTalk said the data showed an "improving trend" on the number of complaints.
The Ofcom report notes that the handling of customer complaints could be improved "across the industry", with only half of those who make a complaint to any provider ending up happy with the result.
The top two reasons for complaining were poor or unreliable connectivity, and issues with broadband speed.
However, the total number of complaints to Ofcom about broadband services fell by 23%. The survey also found:
One company stood out with a sharp rise in complaints. Plusnet's share more than doubled in the last quarter of 2018.
The telecoms regulator notes that, at the time, the firm had experienced problems with a new billing system.
"It's hard for TalkTalk to hide from the fact that their customer service isn't up to scratch," Ofcom's director of market intelligence Iain Macrae told BBC Radio 4's Today programme.
"We absolutely think that the firms need to up their games in the way they handle complaints."
He added that the regulator was encouraged by the fact that the total number of complaints to broadband providers had fallen.
"We're pleased that Ofcom's data again shows an improving trend for the number of complaints received about TalkTalk," a spokeswoman for the firm said.
"And while some of the results in the quality-of-service report are disappointing, we are already seeing more customers than ever staying with us."
The provider would continue to "enhance" the services it offered in response, she added.