The two most successful sports leagues in the world, which bring in billions of dollars in revenue, the biggest corporate sponsors and mammoth audiences every game day, are now sharing an altogether different experience: The National Football League and the English Premier League are enduring startling, double-digital declines in television viewership this season.

Viewership through the first seven weeks of the N.F.L. season is down by 12 percent in the United States while the audiences for Premier League soccer matches this season, which began in August, are down by nearly 20 percent in Britain.

The trends that drove viewers away from other programs on broadcast television in recent years, including cord-cutting and DVRs, had not punished the N.F.L. and the Premier League in the same way. Fans — lots and lots of them — did not seem willing to look away.

They are now, in numbers that are alarming for the leagues, which have grown used to fans’ tuning in in good times and bad, and for the networks, some of which have spent 10-figure sums for the rights to broadcast the games. Viewership for N.F.L. games on CBS on Thursday nights, on NBC on Sunday nights and on ESPN on Monday nights is down by as much as 21 percent.