Valtteri Bottas set for Mercedes and Felipe Massa poised to re-join Williams

Felipe Massa and Valtteri Bottas have raced together at Williams for the last three F1 seasons

Valtteri Bottas' move to Mercedes as Lewis Hamilton's team-mate and Felipe Massa's return to replace him at Williams are close to completion.

Massa, 35, has agreed with Williams to postpone his retirement for at least a year after Mercedes made a pitch for Bottas, 27, to replace Nico Rosberg.

Rosberg announced his retirement after winning the world title last month.

The deal for Bottas to leave Williams for Mercedes is also lined up and is set to be completed in the new year.

Although the deals are not yet finalised, they are as good as done and both will happen over the coming fortnight, BBC Sport has learned.

All the building blocks are now in place - Williams will let the Finnish driver go once a suitable deal with Mercedes for compensation is agreed and Massa has agreed to come back to F1 after announcing his retirement in September.

Now, only the finishing touches need to be put in place, which is expected to happen following the Christmas period.

• Symonds set to leave Williams

• Jost Capito poised to exit McLaren

They are the latest developments in a remarkable post-season period, which has seen a world champion retire, another driver reverse a decision to quit and the futures of several senior technical figures changed.

Mercedes executive director (technical) Paddy Lowe will join Williams to run their team following the departure of their chief technical officer Pat Symonds, which was announced on Tuesday.

Mercedes will replace Lowe with former Ferrari technical director James Allison.

The 48-year-old is on 'gardening leave' from the Italian team but will be free to join Mercedes in the late spring or early summer of 2017.

Williams and Mercedes were both unavailable for comment.

Williams approached Massa after it became clear Bottas was Mercedes F1 boss Toto Wolff's number one choice to replace Rosberg.

Wolff sees him as a close like-for-like replacement for the dependable Rosberg, alongside Mercedes' mercurial superstar Hamilton.

An initial offer of a reduction of 10m euros (£8.4m) in their Mercedes engine bill was rejected but Williams will release Bottas for a better offer, which is expected to be close to their full engine bill of 17m euros (£14.3m).

Lance Stroll will now drive alongside veteran Massa after the Brazilian reversed his retirement decision

Once it had been decided the team could benefit overall from letting Bottas go, Williams needed an experienced driver to replace him.

Their title sponsor, drinks giant Martini, requires at least one driver over 25 for promotional reasons - and Williams' other car will be driven by the 18-year-old Canadian novice Lance Stroll, who is bringing sponsorship said to be in the region of £20m to the team.

Massa was open to reversing his decision because he only quit when it became clear there was no space for him at Williams or any other similarly competitive team.