HBO announced Wednesday that it would start a stand-alone Internet streaming service in the United States in 2015 that would not require a subscription to a traditional television service, a move that intensifies the premium cable network’s growing rivalry with Netflix.

Just hours after HBO unveiled plans for its new service, Netflix announced that its subscriber growth was slower than expected, fueling a 25 percent drop in its share price after the markets closed.

The two companies are battling for a new generation of viewers who increasingly pay only for Internet access. Instead of subscribing to cable or satellite television, this growing audience watches television shows and movies via streaming options like Netflix, Amazon, Hulu, YouTube — and now, potentially, HBO.

Richard Plepler, chief executive of HBO, pointed to 10 million homes in the United States with web service but no traditional cable or satellite television subscriptions — half of which, he said, subscribed to a streaming service.