Valtteri Bottas says Williams still needs to raise its game 'in all areas' if it has a chance of challenging for wins against Mercedes and Ferrari next season.

Though he lost out on fourth position in the overall drivers' standings to fellow Finn Kimi Raikkonen at the final round of the season, Williams once again ended the year with third position in the constructors' standings.

Consolidating its sharp upturn in form from the start of the V6 Hybrid era, though Williams enjoyed a relatively comfortable run to third position in 2015, well ahead of the Red Bull Racing team, it struggled to otherwise match Ferrari in second position.

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Underpinned by a series of operational errors throughout the year, most recently the pit-lane release that led Bottas to clip the rear of Jenson Button's McLaren at the Abu Dhabi finale, Bottas admits there is an overriding sense of disappointment about the season and it will take improvements across the board for it to mount a more convincing challenge in 2016.

"If we want to win we have to raise our game in all areas. We need to make big gains over the winter if we want to fight with Ferrari and Mercedes again. And we also need to minimise the number of mistakes like the ones we did today - that's a fact.

"There were lots of good moments, like a couple of podiums and successful races for us," he continued. "But there were a lot of disappointments too, that we need to learn from, because this is not the best we can do - we can do much better like this as a team. We have to remember the good moments more, of course, but what is the main thing is to learn from the disappointing moments."

Indeed, though Bottas says the dip in form towards the end of the season can be partly explained by Williams focusing more of its efforts on development of the 2016 machine, he still believes the team largely underperformed in the latter races.

"We haven't been making much progress in this later part of the season, we've been focusing more on next year's car, so it was a little bit expected we'd drop back a bit. Still, it's a bit disappointing that we finish the year with a poor race when last year we managed to finish, as a team, on a high."