Williams driver Valtteri Bottas is Mercedes' first choice to replace Nico Rosberg as Lewis Hamilton's team-mate, BBC Sport has learned. But Williams have rejected an initial offer to release Finn Bottas from his 2017 contract. In a separate development, Mercedes technical boss Paddy Lowe is close to finalising a deal to join Williams. Lowe is set to be replaced at the world champions by former Ferrari technical director James Allison. The Mercedes driver conundrum Mercedes Formula 1 boss Toto Wolff has zeroed in on Bottas as his favoured option as a replacement for Rosberg, who retired five days after winning his first world title last month despite having two further years on his Mercedes contract. Wolff has approached Williams with a proposal that he gives them a major reduction in their engine bill - said to be in the region of 10m euros (£8.4m); more than half the total fee - in return for releasing Bottas. Williams also have the option to run Mercedes reserve driver Pascal Wehrlein in Bottas' place. Wolff, who is part of Bottas' management team, sees the 27-year-old as a close like-for-like replacement for Rosberg - quick, a reliable and consistent points scorer, and relatively easy to manage alongside the more mercurial Hamilton. Williams have said no to Wolff's first offer but negotiations are expected to continue in the coming days. Jump media playerMedia player helpOut of media player. Press enter to return or tab to continue. British GP: Valtteri Bottas drives 'Silverstone on ice' Williams' problem with losing Bottas Williams feel they need an experienced contender in one of their cars next season because their other driver is Canadian rookie Lance Stroll. The 18-year-old, who won the Formula Three Euroseries in 2016 and comes with a budget reputed to be at least £20m, has been on an extensive test programme intended to prepare him comprehensively for his F1 debut. But Williams are concerned that partnering him with 22-year-old Wehrlein, who has only one year's experience with the back-of-the-grid Manor team, would be a significant risk to their chances of maximising their championship position. Each place in the constructors' table is worth millions of pounds. There is also a complication with Williams' sponsors, among them the drinks giant Martini, for which having two drivers under 25 does not sit well in promotional terms. German Wehrlein is also in talks with the Swiss Sauber team, which is yet to name a team-mate for Swede Marcus Ericsson. Jump media playerMedia player helpOut of media player. Press enter to return or tab to continue. Up close and personal with F1 driver Pascal Wehrlein Another option for Williams would be to replace Bottas with Brazilian Felipe Nasr, who drove for Sauber in 2015 and 2016. Mercedes are believed to be reluctant to promote Wehrlein to a race seat because they believe he lacks the necessary experience to be a guaranteed performer who can help them retain the constructors' title. But Mercedes' other choices are limited - every other driver with experience is contracted to a rival. One outside possibility if a deal cannot be done with Williams is Carlos Sainz. The 22-year-old Spaniard is under contract to Red Bull next season to continue with their junior team Toro Rosso for a third season, but sources say the drinks company would release him if Mercedes came in for him. This has not yet been discussed extensively within Mercedes and would not be a preferred option, not least because of their difficult relationship with Red Bull. Technical merry-go-round Paddy Lowe Lowe has served as Mercedes' executive director since 2013, after previously working with Williams and McLaren Lowe's departure from Mercedes would break up the technical team that has dominated F1 since 2014. In his role as executive director (technical), he has been in overall control of the sporting and technical sides of the team, with only Wolff in a more senior role. Williams have been courting the 54-year-for some months and sources close to the deal say he is now on the verge of finalising an agreement to rejoin the outfit where he started his F1 career nearly 30 years ago. Lowe has not yet held formal discussions about his future with Wolff, BBC Sport understands. Allison has had talks with Mercedes and is said by senior insiders to have agreed a deal to run the technical and design side of the team. The 48-year-old, who left Ferrari as part of a reshuffle in July, is one of the most highly-regarded aerodynamics experts and design leaders in F1. Sources close to Ferrari say he is on a year's gardening leave and could not join a rival until the summer of 2017. Lowe would also have to serve a notice period of at least six months once he and Wolff have held the talks that will determine his future. Mercedes declined to comment. Allison worked at Ferrari through their dominant era with Michael Schumacher between 2000 and 2004; won two drivers' titles with Fernando Alonso at Renault in 2005 and 2006; and stayed there through its change into Lotus in 2012, before moving back to Ferrari in 2013. Following his split with the Italian team in the summer, Allison has already turned down offers from Williams and Renault. Red Bull are not interested in him. McLaren, who came close to signing him in 2012, have been told that Allison is not available because he is committed to another team - which they believe to be Mercedes.