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 PS13million 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 One, 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.