Valtteri Bottas believes that Williams face a "big challenge" this season, but are hopeful of competing strongly against their Formula 1 rivals.

Mercedes took the constructors' crown in the 2014 campaign, while Williams finished third in the standings.

"We are determined to do better," BBC Sport quotes Bottas as saying. "But now the difficult part starts because to move up from third in the constructors' is not that simple.

"We know it's a big challenge, but we are definitely ready for it. As a team we improved so much last year, and now it's a new year we need to make sure we start at the same level we finished last season, and then build from that."

The season begins with the Australian Grand Prix next month.