HOCKENHEIM, Germany, July 27 - Former champions Williams will switch to Toyota engines next season in a three-year deal they believe can take them back to the top in Formula One.

"This agreement is the cornerstone of Williams's challenge for the world championship," team boss Frank Williams said in a statement on Thursday.

"Toyota is an impressive and remarkable industrial giant, with the most phenomenal reputation for achieving the goals it sets itself. We are proud to have secured their support in our efforts to return to our competitive best."

Williams, who are powered by privately-owned Cosworth this season after separating from BMW, said they would use the same specification engines as Toyota's own team.

The long-awaited announcement, before Sunday's German Grand Prix, will mean the end of Toyota's current agreement with uncompetitive tail-enders Midland.

"Both Williams and...Toyota Racing see significant benefits from a relationship based on robust on-track competition and close off-track co-operation," said the statement.

Toyota motorsport president John Howett added: "We are pleased to be selected as the engine provider to Williams and look forward to establish a strong relationship with them.

"We look forward to both teams using each other as a comparative benchmark from which each can improve its own performance and to competing with each other at the front of the grid," he said.

Williams, owned 70 percent by the team boss and 30 percent by co-founder Patrick Head, have won 113 grands prix since 1979 and nine constructors' championships.

Their last titles were in 1997, when Canadian Jacques Villeneuve also won the drivers' crown.

Toyota entered Formula One in 2002 but have yet to win a grand prix despite having one of the biggest budgets in the sport.

Williams are currently eighth in the constructors' standings with 10 points while Toyota are fifth with 21.

While Williams remain an independent team, the engine agreement marks their return to the manufacturers' fold -- seen as a pre-requisite for success in Formula One -- and could lead to greater cooperation with Toyota in future.