TalkTalk and Sky both achieved a customer score of just 50 per cent, leaving them at the bottom of the Which? ranking.
Customers criticised TalkTalk for the quality of its customer service and technical support and failed to score well in any category, including value for money.
TalkTalk customers were also the most likely to suffer from very slow speeds (27 per cent) and frequent connection dropouts (21 per cent) in the past 12 months. Sky fared slightly better than TalkTalk in most categories, but more than two-thirds of Sky customers surveyed told Which? they were likely to switch broadband provider.
More than one in five Sky customers (22 per cent) also said they had experienced issues with very slow connection speeds.
BT, Sky, TalkTalk and Virgin Media supply almost nine in ten broadband customers, but Which? noted they had languished at the bottom of the ranking for years now.
BT also received a “dismal” 51 per cent, with customers particularly unhappy about the provider’s value for money and 20 per cent of households experiencing problems with very slow speeds or connection dropouts.
Virgin Media fared only slightly better at 58 per cent, but its customers were by far the most likely to complain about price increases at 54 per cent.
The sharpest fall in the rankings came from Vodafone, which earned a customer score of 58 per cent having been joint fourth in the watchdog’s spring 2018 survey.
Some 71 per cent of those surveyed said they had been with their provider for more than three years.
Which? has previously found that customers who have been with their broadband provider for a long time without haggling for a better deal are the most likely to be overpaying – potentially by hundreds of pounds a year.
Natalie Hitchins, Which? head of home products and services, said: “It’s outrageous that the biggest providers are still letting their customers down with shoddy broadband, especially when we know that long-standing customers are the most likely to be overpaying.
“Anyone who is unhappy with their current provider should take back control and switch to a better deal – you could get better service and save hundreds of pounds a year.”
A TalkTalk spokesman said: “While these results are disappointing, we are already seeing more customers than ever staying with us as we continue rolling out major service improvements.”
Virgin Media said: “We continue to invest around £1 billion a year on our network, as well as our products and services such as new in-home Intelligent WiFi features. We are committed to delivering a first-class service to our customers.”
Which? surveyed 8,063 members in January.