Valtteri Bottas will drive alongside Lewis Hamilton for Mercedes in the 2017 Formula One championship after leaving Williams, who announced the long-anticipated move on Monday. The 27-year-old Finn is stepping into the seat unexpectedly vacated by Nico Rosberg when he retired after winning the title last year.

Bottas will be replaced at his former team by Felipe Massa, who has come out of retirement to return to Williams to retake his position at short notice.

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Mercedes were caught by surprise by Rosberg’s announcement which, coming at the end of the season, left the team in a difficult position with almost all of the current F1 drivers already under contract elsewhere. The Bottas deal would likely have been facilitated because Mercedes’s head of motorsport, Toto Wolff, is part of the driver’s management group and Williams use Mercedes engines.

The length of his contract has yet to be revealed and, with drivers such as Fernando Alonso and Sebastian Vettel concluding their obligations with McLaren and Ferrari at the end of this season, it is likely Mercedes have kept their options open with a one-year deal and the option of extension.

The Finn has undoubted talent but although Wolff has made it clear Hamilton and Bottas will be treated as equals – as was the case with Rosberg and Hamilton – the British driver will be the de facto No1 in the team. This then will be the first time Hamilton has enjoyed real seniority over a team-mate in his career, with the exception of two years alongside Heikki Kovalainen, having previously been partnered with Alonso, Jenson Button and Rosberg. But Bottas will be not be easy to dominate nor can he be expected to be a pushover on the track, however.

He opened his career in karting, before graduating through Formula Renault into Formula Three and then GP3. He won the title in the latter series in 2012, showing skilled teamwork, maturity and racecraft to develop an initially poor ART car into a championship-winner and was test driver for Williams that year as well before taking a full drive in 2013. Since then he has shown considerable promise, outscoring his team-mates in all four of his seasons with the team.

The 2013 Williams was uncompetitive but given a car with real pace in 2014, Bottas proved more than capable of stepping up. Although he has yet to win a race, that year he scored six of his nine career podiums from 77 starts.

It included a magnificent performance to go from 14th to second at the British Grand Prix, including passing Kevin Magnussen and Jenson Button round the outside at Stowe corner – moves that required precision judgement and no lack of confidence in his own ability. It was a season when Claire Williams, the deputy team principal, believed he showed potential as a future world champion.

He has proved uncompromising as well and is outspoken on team radio when believing he is being held up by slower team-mates. Some work is still to be done, occasional unforced errors have compromised some of his drives as have moments of frustration and Hamilton’s greater experience in competing at the front gives him a clear advantage across the garage in this regard.

“I’m ready to work hard, to prove myself to the team and to prove my skills,” Bottas said. “It’s going to be a challenging season and joining a new team makes it more work than normal. But I’m 100% ready for that. I always set the bar really high, so my target is to perform from the first race.”

The team, looking for a fourth constructors championship in a row, will also be looking for strong performances from the off. “Valtteri is a no-nonsense guy: down to earth, straightforward and very focused. Pretty Finnish, to be honest, and a great fit for us,” Wolff said. “Now it’s time for the next level, to see how he can step up to challenge for race wins and for Championships. We have confidence in Valtteri’s ability and it’s one of the reasons that he is the driver we set our sights on this winter.”

Sauber’s decision on Monday to sign Pascal Wehrlein, the German Mercedes junior driver, completes the full team lineups for the 2017 season. Mercedes also announced that Rosberg has accepted an ambassadorial role with the team for 2017.