BT Openworld and Broadband are proportionally more complained about than any other UK internet service providers, according to official data.
Oftel, the telecoms industry regulator, received 0.7 complaints per 1,000 internet customers for BT, which was more than double the market average.
Pipex and Tiscali were also worse than average among leading ISPs.
There were fewest complaints about services provided by AOL, Freeserve and Telewest.
Overall, 4,700 internet users took complaints to Oftel in the period from April to the end of September.
The most common net consumer gripe was incorrect billing, followed closely by erratic connection.
A BT spokesperson said the company's internal surveys indicated customers satisfaction levels were rising.
BT's fixed-line phone service fared much better in the survey.
Oftel received 0.4 complaints per thousand BT customers - below the market average.
Npower, better known for supplying electricity, received 1.6 complaints per thousand customers about its fixed-line telecoms service. This was worse than any of its rivals.
In the mobile market, all the major network operators - O2, Vodafone, T-Mobile and Orange - came out better than average.
Virgin Mobile was the least complained about network, with less than 0.1 customers per thousand taking their case to Oftel.
Oftel received 51,750 complaints in total from March to the end of September.