Comcast on Friday dropped its $45 billion merger with Time Warner Cable after Washington regulators raised concerns that the combined giant would hold too much sway over the rapidly evolving television and entertainment industries, according to people familiar with the matter.

The decision caps a spectacular collapse for one of the biggest deals ever to come before federal officials. The merger would have combined the two largest cable providers in the country and would have put more than half of all high-speed Internet customers under one company.

The move by regulators to throw up roadblocks shows that the government has grown concerned about massive media conglomerates bigfooting rivals that are finding success by streaming content over the Internet, analysts said. And after years of approving a wave of mergers in the industry — including that of Comcast and NBC Universal in 2011 — federal officials are taking a new tone, they said.

“It sends a clear signal to the market that the dominant cable gatekeeper cannot stand in the way of skinny packages from programmers, online offerings, the kinds of things we’ve been hearing about,” said Gene Kimmelman, a former antitrust official at the Justice Department who is now president of the consumer advocacy group Public Knowledge. “People beginning to develop new services will now explode — services from Apple, Amazon, Sony, Google, the new offerings from Sling TV, ESPN on wireless devices.”

For Comcast, the opposition from Washington was a massive blow.

Few companies have had more success in persuading regulators to allow them to grow than Comcast, which evolved from a small cable television operator in rural Mississippi into a global media and telecommunications juggernaut. Merger after merger was approved over the decades much with the help of an army of lobbyists in Washington and in cities across the nation. Top executives including chief executive Brian Roberts golf with President Obama and are perennial big donors to political campaigns.

Excluding the millions of dollars that Comcast poured into advertising the benefits of the deal in major media outlets, the company spent $17 million in lobbying last year and had a team of 128 lobbyists, according to the Center for Responsive Politics.

Comcast and Time Warner Cable declined to comment for this report. The people who were briefed on the companies’ plans spoke on the condition of anonymity because no formal announcement has been made at the time. The deal was officially called off early Friday. Bloomberg News first reported that the companies had decided to cancel the merger.

The deal was originally proposed by Comcast and Time Warner Cable in February 2014. At the time, financial analysts predicted that the merger would be easily approved by the Justice Department and the Federal Communications Commission. The companies generally do not compete with each other in the same regions of the country and where they do, they offered to shed millions of customers to direct rivals.

But the argument set off a firestorm from consumer advocates and Democratic lawmakers who argued that, in the new media landscape, geographic dominance has become far less of a concern. They added that the new combined company — as a dominant provider of high-speed Internet and cable and a creator of content through NBC Universal — would have far too much leverage over a wide range of online streaming companies, Web sites, programmers and others.

That regulators would find such arguments persuasive hints at a change in thinking among antitrust officials in Washington, analysts said.

“Comcast spun a story about this purely being a horizontal play and that Time Warner doesn’t compete in the same Zip codes — it’s a neat and surgically narrow interpretation. But that’s an unrealistic and inaccurate view of what’s really going on here, and Justice and the FCC know that they are using tools at their disposal to protect competition broadly,” said Diana Moss, president of the American Antitrust Institute.

Indeed, Justice officials investigated complaints that Comcast used its influence over the media industry to disadvantage rival telecom and satellite firms, according to a person familiar with the matter. Comcast allegedly violated its 2011 merger agreement when it acquired NBC Universal to be a passive co-owner of the streaming service Hulu.

In 2013, when co-owners Walt Disney Co. and 21st Century Fox wanted to sell Hulu, Comcast persuaded the media partners to hold off on the sale even though AT&T and DirecTV were bidding as much as $1 billion for the streaming service, according to the person. Justice officials were concerned that the action demonstrated Comcast’s enormous sway over the entertainment business and its ability to use one corporate unit to disadvantage rivals in another industry.

“Even though there was a very good bid out there, Hulu was taken off the block, and that raised questions,” said the person familiar with the federal inquiry, who spoke on the condition of anonymity to discuss the matter freely. “They suspect Comcast injected itself in a way that was against their merger conditions.”

That power would have increased had Comcast become the nation’s most dominant provider of high-speed Internet, critics of the deal said. Watching TV over the Internet has grown more popular in the year since the deal was proposed, as HBO, CBS and others gave in to demands by consumers who wanted to watch their favorite shows online at a lower cost than what is charged for massive bundles of cable channels.

Such questions were brought to Comcast’s attention in separate meetings this week at the Justice Department and at the FCC.

After its meeting with Comcast on Wednesday, the FCC prepared to hand off its review to an administrative judge — a rare move only for deals the agency is ready to block. At that point, analysts said, Comcast had little choice but to walk away.

“It is a tribute to [FCC Chairman] Tom Wheeler for demonstrating willingness to take on the politically powerful cable industry,” said Andrew Schwartzman, a law scholar at Georgetown University. “There has been, and there would be, a lot of political heat for doing this.”