Williams has confirmed it will end its Formula One engine deal with Toyota at the end of the 2009 season.
The independent has used Toyota engines since 2007 and is now expected to use Cosworth units from next year.
The move is likely to raise further doubts about Toyota’s future involvement in the sport as it will now supply no other teams with engines next year apart from its own works outfit.
Toyota motorsport boss Tadashi Yamashina added: "We have enjoyed a productive working relationship with Williams and I believe both companies have benefited from that partnership. We respect the wishes of Williams to look for other opportunities and wish them well for the future."
Team boss Frank Williams praised the Japanese outfit for the support it had received over the past three seasons.
"At a time of great difficulty for us as a team, Toyota stepped forward and agreed to supply us with engines for 2007 through to the end of 2009," he said.
"Toyota has been a very decent partner who have always respected our independence and provided us with a first-rate technical product. We are proud that we have been able to work with such a great company and we are grateful to the team in Cologne for their support.
"I would like to single out for our thanks the Toyota engineers and technicians who have worked within our race team tirelessly for the past three years."