Mercedes reserve driver George Russell has joined Williams for the 2019 season after signing a multi-year deal.

The 20-year-old, who has driven in practice sessions for Force India and also carried out testing work for Mercedes, will line up on the grid for his Formula One debut at the Australian Grand Prix in March next year.

Russell leads the Formula 2 title race by 37 points with only one round of the season remaining and is the reigning GP3 champion.

The Brit, who joined the Mercedes junior programme at the start of last year, will fill one seat for Williams, who are yet to confirm the identity of Russell's team-mate.

"It is a huge honour to join a team of Williams' prestige and heritage. Formula One has been a life-long dream." said Russell.

"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."

Deputy team principal Claire Williams revealed Russell has been on Williams' radar for a while.

"I am delighted to announce that George Russell will be joining Williams from the 2019 season. We have always tried to promote and develop young talent at Williams, and George fits that ethos perfectly." she said. 

"He is already highly regarded in the paddock and a driver whose career we have been watching for a while.

"In the time we have spent with him so far, we believe that he will be a great fit for our team; his commitment, passion and dedication is exactly what we need to drive the positive momentum building at Grove as we focus on the future.

"We are extremely excited to welcome George and work with him moving forward."

Sergey Sirotkin and Lance Stroll are Williams' current drivers, though the latter is expected to join Racing Point Force India after his father, Lawrence, led a buy-out of the team.