Ake, 22, spent time on loan with the club last season and becomes their third major signing of the summer, following the arrivals of Asmir Begovic and Jermain Defoe.

Sources told ESPN FC on Wednesday that Ake, who can play left-back, centre-back or defensive midfield, would leave Chelsea in a deal worth £20 million.

"I had a great time here last season so I'm really happy to be back," Ake told the club's official website. "It's an exciting challenge. Chelsea is a great club and I learned so much there but this is a new step and the right place for me to develop even more.

"The fans haven't seen the best of me yet. I always set the bar high so I know there is a lot of things I still have to work on. With the help of the manager here I know I can keep improving and become a better player."

Bournemouth manager Eddie Howe added: "I am delighted that we have completed the signing of Nathan on a permanent basis. He is someone we know all about, having had a very successful loan spell here last season.

"Nathan is an outstanding young player with a fantastic attitude and a great desire to learn and develop. He has a very bright future ahead of him and I'm very pleased to say that future is here."

The Blues have already raised around £42m from the sales of Begovic, Juan Cuadrado, Christian Atsu and Bertrand Traore this summer, as they prepare for a spending spree intended to equip Conte with a squad capable of challenging in the Champions League next season.

Tiemoue Bakayoko is set to become Chelsea's first new signing once the details of a deal with Monaco are finalised, while Juventus wing-back Alex Sandro has also been heavily linked with a move to Stamford Bridge, and talks are ongoing with Everton over striker Romelu Lukaku.