Satellite TV giant is angry at BT for offering sports channels free to anyone with a BT broadband package.
BT Sport has been dismissed by Sky as a "marketing gimmick" for BT broadband.
BT revealed yesterday that its forthcoming sports TV channels - BT Sport 1, BT Sport 2 and ESPN - would be available free of charge to all BT broadband customers.
"This is all about broadband and BT's latest attempt to stem the flow of customers who've switched in their millions to rivals like Sky over the last few years," said Stephen Van Rooyen, Sky's managing director of sales and marketing.
He told broadbandchoices.co.uk: "BT Sport is not 'free' and customers are smart enough to realise they'll pay for it through more expensive broadband and phone services."
"Our approach is to offer high-quality broadband at great value with the best customer service. For us, sport isn't a marketing gimmick to promote another product. We're long-term supporters and our sustained investment has benefited sport fans and British sport at all levels."
BT Sport has exclusive live broadcast rights to 38 Premier League football games, including 18 of the best quality 'top pick' matches, which, for the first time ever, have not all gone to Sky.
It means in Britain, for the first time in over 20 years, weekly live Premier League football games will be free for anyone to watch on TV, provided they're on BT broadband.