TalkTalk has been rated the worst broadband provider in the UK for customer satisfaction for the third year running, in Ofcom's annual review of telecoms providers.
The company scored below the industry average on several measures, with broadband customers less likely to be satisfied with their overall service and less likely to recommend their provider to a friend.
TalkTalk customers are also more likely to have a reason to complain, less likely to have their complaint resolved on first contact, and less likely to be satisfied with how their complaint is handled.
"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.
Dani Warner, telecoms expert at uSwitch.com, also said that the figures were "particularly unflattering" for TalkTalk, which has "continuously ranked poorly in terms of customer care".
However, a spokesperson for TalkTalk said the data showed an "improving trend" on the number of complaints, and that the company would continue to "enhance" its services.
"We're pleased that Ofcom's data again shows an improving trend for the number of complaints received about TalkTalk," a spokesperson 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."
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Ofcom interviewed 19,000 UK adults in total for its report - 9,517 about complaints handling, 3,378 about customer satisfaction, and 6,399 about reasons to complain.
Overall, 83% of broadband customers are satisfied with their service (up from 80% last year), while 13% had reason to complain in 2018 (down from 15% in 2017).
Ofcom said that complaints handling could be improved across the industry – with only around half of those who made a complaint claiming to be satisfied with how it was handled.
Sky broadband customers are less likely than average to have reason to complain, and more likely to be satisfied with how their complaint is handled, according to Ofcom.
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However, complaints about Plusnet – owned by BT – more than doubled in the last quarter of 2018. In that period, the company experienced issues with a new billing system it launched.
Responding to the report, consumer watchdog Which? said suppliers that are lagging behind should be doing much more to tackle these issues.
"Whether it is reliable connections, customer service or complaints handling, these figures show that some providers have the upper hand when it comes to giving customers what they want," said Natalie Hitchins, Which? Head of Home Products and Services.
"If you think you are overpaying or not getting the level of service you expect from your provider, shop around and switch as soon as you can."
