The U.S. economy, now officially in recession, may be in the midst of the longest slump in the post- World War II era as job losses mount and credit dries up.

The economic slump began in December 2007 when payrolls reached a peak, the business cycle dating committee of the National Bureau of Economic Research, a private, nonprofit group of economists based in Cambridge, Massachusetts, said yesterday. The last time the U.S. was in a recession was from March through November 2001, according to NBER.

“We’re going on 12 months already, and we’re just getting started,” said Stephen Stanley, chief U.S. economist at RBS Greenwich Capital in Greenwich, Connecticut. “We’re looking at some pretty severe numbers for the fourth quarter, and the first quarter of 2009 will be pretty bad as well. The economy isn’t going to turn around definitively until the credit markets unclog.”

The NBER designation means the U.S. was the first country to have slipped into a contraction. While definitions differ, the economies of both the euro area and Japan fell into a slump in the second quarter of this year, making it the first simultaneous recession in the three regions in the postwar era.

The longest economic slumps since 1945 were the 16-month downturns that ended in March 1975 and November 1982. The Great Depression lasted 43 months, from August 1929 to March 1933.

“This may be referred to as the Great Recession,” because of its length, said Norbert Ore, chairman of the Institute for Supply Management’s factory survey. “It looked like we were headed for a shallow recession earlier in the year because of higher energy prices. With the meltdown in the financial sector, it has become something more serious.”