Lotus have confirms that Grosjean will partner Raikkonen 2013 Williams Chief Operations Engineer Mark Gillan has left the team, it was reported on Monday.
Autosport said that Gillan, who joined Williams in 2011, had left them to "spend more time with his family".
With a background in aerodynamics, Gillan had previously worked in Formula 1 for McLaren, Jaguar/Red Bull and Toyota.
The Northern Irishman also held a professorship at the University of Surrey, and it was from there he was lured by Williams in September 2011.
The team, nine-times World Constructors' Champions, were in the process of shaking up their technical department at the time as they sought to boost their competitiveness.
Williams finished a lowly ninth in the 2011 standings but with Gillan joined by the likes of new Technical Director Mike Coughlan and Head of Aerodynamics Jason Somerville, they enjoyed a better season in 2012.
Results were still inconsistent, meaning that Williams finished only one place higher in the overall standings.
However, the team's Renault-powered FW34 was much-improved upon its predecessor and with Pastor Maldonado driving, the team scored their first win since 2004 in the Spanish Grand Prix.
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