Williams have confirmed the appointment of Mercedes junior driver George Russell to a 2019 race seat on a "multi-year" deal. Williams' move for Russell could spell the end of Esteban Ocon's hopes of securing a 2019 race seat, with Sergey Sirotkin expected to retain his place in the team.
Russell is closing in on the Formula 2 title in 2018, requiring just a podium finish in the final round of races in Abu Dhabi to secure the title.
If he is crowned, Russell will be unable to stay in F2, as champions are not permitted to defend titles, potentially pushing Mercedes to elevate him into F1.
At least one seat had been expected to be up for grabs at Williams, with Lance Stroll near certain to join his father Lawrence at Force India in 2019 - which is set to leave Ocon without an F1 seat.
Russell also holds titles in GP3 and British Formula 4 and set a new outright circuit record at the Hungaroring in a testing appearance for Mercedes this year.
Russell said: "It is a huge honour to join a team of Williams’ prestige and heritage. Formula One has been a life-long dream.
"From watching the races when I was a child, it feels surreal that I will now be lining up on the grid, alongside drivers whom I have admired for years.
"I would like to thank Claire and everyone at Williams for their faith in making this decision, as well as Toto and the team at Mercedes for their fantastic support throughout GP3 and Formula 2. I am incredibly excited to start working with everyone at Grove and to take my first steps as a Formula One race driver.
"I cannot wait for Melbourne next year and to join Williams at the start of what we all believe will be the start of an exciting new journey."
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