Williams has confirmed that it will part company with Toyota at the end of the 2009 season.
The move ends an engine supply deal that began at the start of 2007, and had been anticipated for several weeks, with senior Williams figures openly admitting that the team was reviewing its engine options for 2010.
Team boss Frank Williams thanked Toyota for its support over the last 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."
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."
Williams's statement said that its new engine supplier would be announced "in due course."
The team has been linked with both Renault and Cosworth for next season.