Regulator Ofcom said the complaints about BT TV over the July-September quarter peaked in August, with almost half relating to BT Sport, which launched that month.
The telecoms giant has said it doubled its number of new broadband customers over the period to 156,000 -with many lured by a package including 38 top-flight football matches a season on its TV service.
It said in October that it had signed up more than two million customers for its sports channels.
But Ofcom said it received 0.56 complaints about BT TV per 1000 subscribers, more than double the 0.26 figure from the previous quarter - a figure which was already several times worse than pay-TV rivals Sky and Virgin Media. It said overall complaints about BT TV were mainly related to service issues and complaints handling.
BT said: "BT apologises to BT TV customers who suffered poor service during the summer. The main reason for this was the unprecedented volume of interest in BT Sport and our efforts to provide availability on multiple TV platforms in a very short space of time."
BT has paid £738 million for a three-year deal to show Premier League games as part of an aggressive strategy to respond to Sky, with both competing to sell bundled packages that include broadband, telephone and TV services.