While the biggest services are located in China, it takes a fan, often in the United States, to upload the actual stream for distribution to the wider Internet. This is done by using a PC-tuner card, a $50 device that connects a television to a computer, or by uploading the stream from a legitimate online video subscription to a peer-to-peer network.

As technology advances, the problem is likely to become worse as the quality of the picture becomes better. “This can only become a more enjoyable experience,” said Christopher Stokes, the chief executive of NetResult, a company in Britain that consults with the leagues and acts as a sort of Web detective in locating digital pirates.

A recent unpublished report for the Organization for Economic Cooperation and Development in Paris, the first major study of the issue, concluded, “This poses a major issue for the leagues and organizations who own and control sports as the sale of broadcast rights represents a major source of revenue, enabling the sports leagues to thrive and continue, from grassroots level to professional league.” The organization helps developed nations tackle mutual economic problems.

David Price, the head of piracy intelligence at Envisional, a consultant that helped prepare the O.E.C.D. report, said: “I think it’s different than looking at movies or music. You might not go to the cinema, but you’ll buy the DVD. With sports, they very much have this one shot to get you to watch the game. If suddenly there is a way to get that live transmission for free, then there is a real threat to their business.”

When leagues try to shut down the pirates, executives can find themselves immersed in an endeavor rife with international intrigue: investigators for the Premier League, England’s top soccer league, chased an offender to Cyprus to serve him court papers, according to Oliver Weingarten, a lawyer for the league.

On a recent morning at the Manhattan offices of Major League Baseball Advanced Media, M.L.B.’s digital unit, an executive started a laptop and went to a live feed of a soccer game on ESPN Hong Kong that was carried by TVants.com. M.L.B. officials say TVants.com, based in China, is one of the biggest providers of live game feeds.