Williams will sever their partnership with BMW and use Cosworth engines next season, the Formula One team said on Monday.
Former champions Williams said they will also invest in a wide-ranging partnership with the privately-owned British-based engine manufacturer.
"The evidence of Cosworth's V8 progress we have seen so far validates completely the expectation we have of enjoying a class-leading package given next season's engine rule changes," said team boss Frank Williams.
Formula One is moving from three litre V10 engines to 2.4 litre V8s next year.
Williams have been powered by BMW since 2000 but the German manufacturer has decided to buy Sauber and enter their own team from the start of the new season.
Williams have Australian Mark Webber under contract and expect Briton Jenson Button to join from BAR in 2006, although the 25-year-old wants to stay where he is.
