The move paves the way for George Russell’s graduation to Mercedes to partner Lewis Hamilton for next season, which is expected to be confirmed by the reigning world champions in the coming days. 

Bottas has scored nine victories during his five-year stint with Mercedes and played a key role in helping the German marquee secure consecutive world titles to maintain its unbeaten streak in the V6 hybrid era. 

Despite twice finishing runner-up to Hamilton in the world championship, Mercedes has opted to promote its protege Russell after a difficult and so far winless campaign for the 32-year-old Finn.

Bottas' signing is a coup for Alfa Romeo and he will effectively take over from his compatriot Raikkonen in becoming team leader for the Swiss-based squad heading into F1’s rules revolution. 

“A new chapter in my racing career is opening: I’m excited to join Alfa Romeo Racing ORLEN for 2022 and beyond for what is going to be a new challenge with an iconic manufacturer," said Bottas.

"Alfa Romeo is a brand that needs no introduction, they have written some great pages of Formula One history and it’s going to be an honour to represent this marque. The potential of the setup in Hinwil is clear and I am relishing the opportunity to help lead the team forward up the grid, especially with the new regulations in 2022 giving the team a chance to make a leap in performance.

"I’m grateful for the trust the team has put in me and I cannot wait to repay their faith: I’m as hungry as ever to race for results and, when the time comes, for wins. I know Fred [Vasseur] well and I am looking forward to getting to know the rest of the team I am going to work with, building relationships as strong as the ones I have at Mercedes.

"I am proud of what I have achieved in Brackley and I am fully focused on finishing the job as we fight for another world championship, but I am also looking forward to the new challenges that await me next year.”

Alfa Romeo team principal Frederic Vasseur added: “It is a pleasure to welcome Valtteri to the team and we are looking forward to our journey together. With him, we bring to Hinwil a strong team player with experience at the sharp end of the grid.

"Valtteri has been an integral part of a team that rewrote the history books and he has four constructors’ world titles to his name: he is the right driver to help Alfa Romeo Racing ORLEN make a step forward towards the front of the grid.

"Our relationship goes back a long way, to when we worked together in successful campaigns in F3 and GP3, and his talent and skills have been evident ever since, only growing with the time: I’m really looking forward to seeing him put his ability to work for the good of the team.

"The multi-year deal bringing him to us gives both Valtteri and the team the stability we need to build our project at a crucial time for Formula One: we are excited for what the future holds in store.”

Alfa Romeo has not decided whether to retain Antonio Giovinazzi for a fourth successive year. 

The 27-year-old Italian’s current deal is due to run out at the end of the season and his future remains uncertain, despite his consistent performances this year. 

Newly-crowned Formula E champion Nyck de Vries and ex-Red Bull racer Alex Albon have both been linked with the second seat at Alfa Romeo.