A source at Saatchis confirmed that the entire UK business, which it has held since 1993, will move to fellow Toyota roster agency CHI & Partners later in the year.
Saatchi & Saatchi will continue to handle US and some European advertising for the car-maker. The account, worth £1.75 billion globally, is the network's second largest after Procter & Gamble.
The Toyota GB marketing director, Mark Hall, denied the company had any imminent plans to consolidate its marketing into CHI. But he added: "We may change the way we operate in the future."
CHI has increased its hold on the Japanese car marque since 2004, when it won the pan-European launch of the Aygo. It has since won the pan-European Corolla and Yaris briefs, and also runs Toyota's Lexus brand in Europe.
Saatchis' loss of the UK Toyota account will further unsettle the agency, which has been without a chief executive since the departure of Lee Daley in March.
The Saatchis EMEA, Asia and Latin America chief executive, Jim O'Mahony, said the agency had a shortlist of candidates, but that no signing was imminent. He declined to comment on the Toyota loss, and referred calls to Toyota.