Robert Kubica has missed out on a sensational Formula 1 comeback after Williams announced Sergey Sirotkin will drive for them this season.
Kubica, the 33-year-old Polish driver who has not competed in F1 for more than seven years following a life-threatening rally crash in 2010, tested for the British team in the hope of racing again.
But the team, based in Oxfordshire and a winner of 16 combined championships, have instead opted for Sirotkin, the 22-year-old Russian, who has competed in a number of practice sessions, but is yet to make his full Grand Prix debut.
It is understood, however, that Kubica is still in contention for a reserve seat with the British outfit this year.
Sirotkin, who will bring an estimated £13million in backing to Williams, will join forces with Canadian teenager Lance Stroll to form the most inexperienced line-up on the grid.
Stroll, 19, will enter only his second season while Sirotkin replaces Felipe Massa who retired for a second time at the end of last year.
"To say I'm happy and proud to join such a famous team like Williams is an understatement," Sirotkin said. "It took a huge amount of work to get where I am, and I'm really happy and thankful to everyone involved.
"The result of our combined efforts has helped me achieve my dream, and rest assured the team can rely on me to deliver my best."
Sirotkin, who has twice finished third in the GP2 championship, the feeder series to Formula 1, has been a test driver for Sauber and Renault.
He tested against Kubica, in what was seen as a straight shoot-out for the seat alongside Stroll, following the Abu Dhabi Grand Prix in November. He is the third Russian to land a full-time seat in Formula One.
Deputy team principal Claire Williams added: "We have taken our time to evaluate all the available options, and I'm confident Lance and Sergey can deliver the best results for the team.
"The Williams philosophy has always been to promote and develop young talent and Sergey fits right into that ethos.
"Lance has had a record-breaking debut season, and with a year now under his belt, he will be ready to hit the ground running in 2018.
"We have a talented driver line-up for 2018, that we are confident will deliver some exciting results for the team."
The announcement by Williams is the final piece in the driver jigsaw for this year's campaign which gets under way in Melbourne on March 25.