Although some analysts had warned that the merger could raise antitrust concerns, particularly in the New York area, where Continental has a hub in Newark, the government approved the deal in record time. While few expected regulators to oppose the merger, the approval surprised some analysts who had expected the process to take until the end of the year.

“Given the size and the breadth of their networks, the concession to Southwest is a very small price to pay to get the deal done,” said Hunter Keay, an airline analyst at Stifel, Nicolaus & Company.

The rapid timing is a victory for Continental’s chief, Jeffery A. Smisek, who will run the merged company, and who has been pushing to complete the deal quickly. The Justice Department has apparently accepted the position of the airlines, which insisted that their hubs did not overlap. The department said the airlines would combine “largely complementary networks, which would result in overlap on a limited number of routes where United and Continental offer competing nonstop service.”

A sticking point was Continental’s hub in Newark, where new landing and takeoff slots are limited. United and Continental operate 442 daily round-trip flights at Newark. The deal with Southwest “resolves the department’s principal competition concerns and will likely significantly benefit consumers on overlap routes as well as on many other routes,” the Justice Department said.

The all-stock merger will form a coast-to-coast giant with a leading presence in the top domestic markets, including Chicago, Los Angeles and New York, and an extended network to Asia, Europe and Latin America. The combined company will keep the United name and will be based in Chicago.