Tempus-owned CIA, which previously handled Guinness, lost the business after a final two-way pitch against MediaCom, New York, which handles the former Seagram's business that is being bought by Diageo.
The loss of the business comes at a crucial time for Tempus, as WPP Group and Havas Advertising battle it out in a takeover war.
According to reports, CIA pulled out all the stops to hang on to the business, bringing in chairman Chris Ingram and highly regarded consultant Bill Croasdale.
The loss highlights CIA's weakness in the US. The Grey Global-owned MediaCom is said to have triumphed partly because of its significant US network-TV buying power.
For WPP in particular, CIA is attractive mostly because of its strength in Europe. WPP would merge the group with its Media Edge, which is far larger than CIA in North America.
The industry is still waiting for the next move in the takeover battle for Tempus. Yesterday, French group Havas extended its original 541p-a-share offer for Tempus until September 17.
It has yet to make a response to WPP's 555p counter-bid. Analysts believe it will wait until September 10, when WPP's offer document is made available.