London – Paddy Lowe has joined the Williams Formula One team from Mercedes with immediate effect as chief technical officer and shareholder in the group, Williams announced on Thursday.
The team did not say how big a stake Lowe had taken or where it had come from, with details expected to be made known at a later date.
The Briton, who started his F1 career at Williams in 1987, also joins the board of directors and will run the business with deputy principal Claire Williams and chief executive Mike O'Driscoll.
"This is a game changer for us and once again makes us extremely excited about this team's future," said Claire Williams.
The Briton had been on 'gardening leave' from Mercedes, which had already replaced him with former Ferrari technical director James Allison.
At Mercedes, Lowe played an integral role in the team winning the last three drivers and constructors' world championships. Before that he won titles with McLaren and Williams.
Mercedes has won 51 of the 59 races since the 1.6 litre V6 turbo hybrid power units were introduced in 2014.
In his previous six-year stint at Williams, Lowe worked with co-founder Patrick Head and designer Adrian Newey and helped pioneer the active suspension system that took Britain's Nigel Mansell to the 1992 championship.
"It is a huge honour to return in this leadership position and to have the opportunity to become a shareholder," said Lowe. "I am extremely motivated to play my part in bringing success back to the team.
"The vision for the future set out by the Williams Board is powerful and has compelled me to join an organisation committed to building on its unique legacy and to reaching the pinnacle of Formula One once again."
Claire Williams said Lowe's arrival was a boost for a team that last won a Formula One title with Canadian Jacques Villeneuve in 1997.
The current line-up also includes a Canadian, 18-year-old rookie Lance Stroll, alongside Brazilian veteran Felipe Massa following the departure of Finland's Valtteri Bottas to Mercedes in place of retired champion Nico Rosberg.
"Having someone of Paddy's calibre and engineering competence is not only a morale boost for everyone at Williams, but I know it will also significantly support our efforts to return this team back to the front of the grid," she said.
"We want to win races and championships, but to do that you need the best talent in the business. In Paddy we believe we have just that as well as a leader who will drive change."
The season starts in Australia next week.