Leo Burnett lost Gary Topolewski, the chairman and chief creative officer of its Chemistri unit, which was set up by Publicis solely to handle General Motors, the biggest advertiser in the US.
The departure of Topolewski follows that of Chemistri chief executive Patrick Sherwood, who left last week to pursue other career avenues.
A spokesman for Leo Burnett said that the agency was looking into the nature of Topolewski's move to BBDO.
Leo Burnett Worldwide president Tom Bernardin said the agency was "looking carefully at the circumstances surrounding [Topolewski's] recruitment at BBDO".
Topolewski is currently still at Chemistri after agreeing to stay on for a handover period.
It is understood that Leo Burnett will be closely examining Topolewski's contract, which will likely contain clauses that bar him from working on rival brands.
BBDO has denied that it is in breach of any of Topolewski's contract with Leo Burnett, with which the Detroit-based Chemistri is aligned within Publicis Groupe.
At BBDO, Topolewski will take on the role of co-chief creative officer of BBDO Detroit, where he will first focus on the Chrysler brand, as well as working on Jeep and Dodge.
GM and Leo Burnett are concerned that Topolewski's exit could damage plans for new campaigns, including a new Cadillac campaign, and plans for new Pontiac models.
Last year, GM spent $2.49bn (£1.4bn) on advertising across its brands in the US while DaimlerChrysler spent $1.57bn, according to TNS Media Intelligence/CMR.
Publicis through Chemistri, formerly D'Arcy Detroit, handles a large chunk of GM's advertising including GM Service & Parts operations and several global GM operations.
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