LONDON (Reuters) - Williams shareholder and executive director Toto Wolff is leaving to take a new role with Formula One rivals Mercedes, the team said on Monday.

Williams founder Frank Williams (front) poses with non-executive Director Toto Wolff (L-R), co-founder Patrick Head and Chairman Adam Parr beside a Formula One car in London February 9, 2011. REUTERS/Toby Melville

Wolff will relinquish his seat on the company's board but will retain his shareholding, Williams Grand Prix Holdings added in a statement.

Reports in Germany said the 40-year-old Austrian, right-hand man to Williams founder and principal Frank Williams, would take on the role vacated by former Mercedes motorsport head Norbert Haug.

Williams said his responsibilities at the former champions would be shared between members of the board's executive committee.

Frank Williams thanked Wolff for his work, calling him "a key support" when he deputised for him at a number of races last season.

"Positions such as the one offered to him by Mercedes do not come around often. Toto has a long history with them and I certainly was not going to stand in the way of him accepting this once in a lifetime opportunity," he said.

"Toto will retain his shareholding in Williams and will always have a place at Grove but make no mistake; we will fight him hard on the racetrack."

Mercedes have signed 2008 world champion Lewis Hamilton from McLaren for this season and also have Wolff's compatriot Niki Lauda, the three times world champion, as non-executive chairman of their board.

Williams also have a new driver in Finland's Valtteri Bottas, a Wolff protege.

Wolff's Scottish wife Susie still has a year remaining on her contract as a development driver for Williams and also raced in the German Touring Car (DTM) championship with Mercedes.

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