PHD is expected to take the reins on the new Adidas business after the World Cup.
Adidas uses Carat for the rest of its £50 million European business.
The new PHD appointment counters speculation that the sports advertiser would centralise into Carat.
Neil Simpson, Adidas' head of global brand concepts and advertising, said: "Carat is our global media partner but not in the UK, where we have a long-standing relationship with PHD. It's a case of 'if it ain't broke, don't fix it'."
Carat also handles Adidas in the US, having won the media planning and buying from Leagas Delaney in 2001.
Adidas has long had a media arrangement with PHD in the UK, having re-appointed the agency to its UK TV buying account in a head-to-head pitch with Carat in 1999.
Adidas called a global review of its creative account last autumn. Leagas Delaney pitched with DDB as its international partner while 180 paired with TBWA.
No one at PHD was available for comment as Campaign went to press.