BTopenworld is the most complained about internet service provider (ISP) in the UK, according to telecoms watchdog Oftel.
The regulator's figures indicate that the ISP receives 1.6 complaints per 1,000 customers, compared to an industry average of 0.4.
AOL came out on top, with less than 0.1 complaints per 1,000 customers.
NTL did well, but was the most complained about fixed line provider with 0.8 complaints per 1,000 customers.
Being unable to get through to a company was the most common gripe, followed by ignored complaints and a failure to respond.
The third most common was contracts that limited the number of hours users could access the internet each month.
BTopenworld said that it is unhappy with its record and is seeking to improve. It told the BBC that many of the calls were made as broadband interest skyrocketed due to a drop in the price, and that the ISP had struggled to cope.
Oftel received nearly 7,000 complaints about ISPs between April and September.
David Edmonds, Director General of Telecommunications, said: "There is a strong incentive on the companies to quickly address any problems faced by their customers.
"For the first time we have included ISPs in our complaints publication because of the increasing number of complaints received."
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