Updated from Wednesday, Oct. 29

As expected, antitrust regulators at the Justice Department have approved the merger between Delta Air Lines ( DAL) and Northwest ( NWA).

Hours after receiving antitrust clearance, the airlines officially closed the merger.

The deal, announced in April, will create the largest airline in the world, surpassing AMR ( AMR), the parent of American Airlines. The carrier will be based in Atlanta and retain the Delta name.

Currently, Delta is the second-largest carrier, while Northwest is fifth. "We're pleased the DOJ has decided not to challenge the Delta-Northwest merger," said Delta spokeswoman Betsy Talton.

Northwest, founded as a mail carrier in 1926, will disappear as a separate entity. Delta will acquire its shares in a deal valued at about $2.6 billion.

Observers had widely expected the merger to be approved. "After a thorough, six-month investigation, during which it obtained extensive information from a wide range of market participants -- including the companies, other airlines, corporate customers and travel agents -- the Justice Department's antitrust division has determined that the proposed merger between Delta and Northwest is likely to produce substantial and credible efficiencies that will benefit U.S. consumers and is not likely to substantially lessen competition," the division said in a prepared statement.

"The two airlines currently compete with a number of other legacy and low cost airlines in the provision of scheduled air passenger service on the vast majority of nonstop and connecting routes where they compete with each other," the regulators said.

The merger's principal opponents have been the International Association of Machinists, the largest airline union, and the Association of Flight Attendants. Both unions represent workers at Northwest, but not at Delta, and now face union elections that will determine whether workers want to maintain representation.

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