WASHINGTON — In a major victory for the broadcast television industry, the Supreme Court ruled that Aereo — the start-up tech firm that distributes local television signals over the Internet — is illegal.

The 6-3 decision in the case of ABC vs. Aereo is likely to sound a death knell for the tiny company that launched in 2012 as a low-cost alternative to high-priced cable and satellite television. It promised to change the way people watched TV.

Broadcasters — led by CBS, Fox, ABC and NBC — countered that the only thing Aereo was trying to change was having to comply with the Copyright Act.

If Aereo was found to be legal, broadcasters feared that consumers might cut the cord to their pay-TV service, which would mean they would get less money for their signals. Another concern was that cable and satellite operators, increasingly vocal about the rising costs of carrying broadcast signals, would use Aereo or a similar technology to get around those so-called retransmission fees.

But cable and satellite operators also had reason to be wary of Aereo. As monthly TV bills keep rising, in large part because of the cost of sports programming, consumers could opt out with a combination of Aereo and streaming services such as Netflix and Amazon. Such a move might mean having to wait until their favorite shows ended up on Netflix, but it would also save a significant amount of money.

“We think Aereo was on to something by filling a need for low-cost, flexible viewing options,” said Ellen Bloom, a senior director at Consumers Union. “As cable prices keep skyrocketing, the consumer demand will continue to grow for more personalized, affordable ways to watch television.”