Williams have decided to switch to Toyota engines next year with team sources telling autosport.com they have agreed a three-year deal with the Japanese car manufacturer.
Talks between Williams and Toyota have been ongoing for several months while the Grove-based team weighed up whether to switch engine suppliers.
Subsequently, the possibility of linking up with a major carmaker proved the deal clincher and will see Williams end their current partnership with Cosworth after just one season.
Team sources have made it clear, however, that Williams's decision is not because they were unhappy about Cosworth's work this year.
"Cosworth's performance on the budget they have is remarkable," said the source.
It is understood that an official announcement on the deal between Williams and Toyota would be made at the Canadian Grand Prix later this month.
Toyota, however, have said the contract has not been signed yet, although a company spokeswoman told autosport.com the deal would most likely to be inked soon.
That contract is expected to run for 2007 and 2008, with an option for a third season in 2009.
Williams and Toyota technical staff have held meetings regarding the technical issues of the partnership since the season-opening Bahrain Grand Prix, just in case a commercial deal was struck.
Team boss Frank Williams has made no secret of the fact this season that his team were looking at other engine options, and explained that his final decision would not just be made on who had the most powerful power-unit.
"The engine choice is both technical and commercial," Williams said at the San Marino Grand Prix. "Who has got the best chance of winning, versus what else comes with the engine?"
A decision by Toyota to supply Williams would mean that Midland will have to seek an alternative engine supplier for next season, with Cosworth being an obvious candidate.
John Howett, team principal at Toyota, said at the Monaco Grand Prix that he felt it was not ideal for his team if their customer team were fighting at the back of the field.
"For sure it would be better for us to have a more competitive team as partner," he said. "But at this stage, no one can say what will happen to Midland anyway."