ATLANTA (AP) -- After months of speculation and negotiation, Delta Air Lines and Northwest Airlines say they're planning to merge into the world's biggest airline.

But there are several hurdles before the carriers could combine into one proposed giant, Delta Air. They'll need antitrust approval from federal regulators and the support of their labor force.

An attempted merger of United Airlines and US Airways fell apart amid antitrust concerns in 2001. And two of Northwest's largest unions are already signaling their opposition.

If the share-swap becomes final, Delta shareholders get a bigger company, while Northwest shareholders get a 16.8 percent premium over yesterday's closing stock prices. Based on those prices, the deal values Northwest at more than 3.6 billion dollars.

Shares of both companies fell in morning trading after rising on premarket dealings. Northwest shares fell 59 cents, or 5.3 percent, to $10.63 in morning trading while Delta shares lost 98 cents, or 9.4 percent, to $9.50.