The Volkswagen-owned Seat had been one of Bates' biggest accounts and, following the takeover of Cordiant by WPP, Sir Martin fought hard to persuade VW to keep the business with WPP.
WPP handles advertising for not only the main Ford brand, but also Jaguar, Volvo, Range Rover and some of Mazda, in which Ford owns a stake.
However, Ford, WPP's biggest client, was known to be unhappy with sharing its global advertising agency with another major car manufacturer, although WPP's Red Cell network does handle work for Fiat and Alfa Romeo, which was also reported last year to be in talks with Grey Worldwide over its estimated £50m pan-European creative account.
As well as objections from Ford, Germany's VW was also known to be uncomfortable with its Spanish motor group staying with WPP.
One possible solution to the problem for WPP was to leave Bates intact, an option that was ultimately rejected.
Grey Worldwide was the favourite agency to pick up the business and speculation has been rife for several weeks that Seat would move the account.
Grey Global Group chairman Ed Meyer is known to have flown to Barcelona in July for talks with Seat chiefs.
Meyer's meeting followed a conference in Barcelona in June that was attended by Seat's marketing directors in key markets to resolve conflict issues that could arise from a WPP acquisition of Cordiant.
A spokesman for Seat said then that it was reserving its position. "We're happy with the people and the service we're getting. We'll have to wait and see how things turn out."
The win will see Grey open offices in Barcelona, where Seat is headquartered.
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