The advertising agency Campbell Ewald will relocate its headquarters from Warren to the former J.L. Hudson Co. warehouse attached to Ford Field, a source familiar with the matter told Crain's today.

The announcement will be made at a 1:45 p.m. press conference Wednesday at the warehouse space, which was converted into commercial offices when the football stadium, home to the Detroit Lions, opened in 2002.

Mayor Dave Bing's office announced the press conference, about a new corporate office opening in Detroit, on Tuesday morning while not saying which the company is involved.

Campbell Ewald President Bill Ludwig declined to comment. He previously has told Crain's that the agency has been searching for a new office for some time but did not confirm any locations.

The agency has been in its current location since 1978.

The move is thought to involve several hundred of Campbell Ewald's approximately 400-plus employees. It's unclear whether the rest would move downtown, remain in Warren or go to another space.

Space in the old Hudson warehouse leases for $21 to $23 a square foot, according to the stadium's website. The warehouse, in use from 1913 to 1995, was turned into 230,000 square feet of Class A commercial office rental space as part of the $500 million stadium project. Also in the building is the law firm Bodman PLC.

Campbell Ewald was best known for being Chevrolet's ad agency, a relationship that ended after 91 years in April 2010 when the account was shifted elsewhere.

At one point a few years ago, Campbell Ewald had more than 1,200 staffers. Even with a successful effort to diversify the client roster to not rely on any one major client, the agency has 450 fewer employees today.

Campbell Ewald had estimated U.S. revenue of $121 million in 2011 and is the nation's 66th-largest ad firm by that metric, according to the Advertising Age Data Center's most recent industry rankings. That's down from $157 million in 2010 and a dramatic drop from 2007, when Campbell Ewald brought in an estimated $239 million.

Chevrolet reportedly accounted for $30 million in revenue and $600 million in media billings in 2009, Ad Age reported.

The agency's largest account for years has been the U.S. Navy, which it first won in 2000 and defended during mandatory reviews in 2005 and 2009. Campbell Ewald handles the service's recruiting marketing.

Its client roster also includes Alltel Wireless, Carrier, Consumers Energy Co., Ghirardelli, Kaiser Permanente, Olympic Paints and Stains, U.S. Mint and USAA. It also retained some General Motors Co. work after losing Chevy.

Last month, Campbell Ewald won the creative account for Cambridge, Mass.-based Zipcar, the car-sharing service now owned by Avis Budget Group.

The agency also has offices in Los Angeles, San Antonio, Miami and Washington, D.C.

Campbell Ewald is owned by the New York City-based Interpublic Group of Cos.