Four-time defending champion Sebastian Vettel will leave Red Bull at the end of the season, with the team expecting him to join Ferrari.

Vettel told Red Bull chief Christian Horner on Friday night that he intended to leave, and the team made the formal announcement on Saturday ahead of qualifying at the Japanese Grand Prix.

Red Bull immediately named a replacement, promoting Daniil Kvyat from its associated team Toro Rosso to join Daniel Ricciardo in a youthful 2015 lineup.

Vettel had been under contract with Red Bull for next season, but had also been strongly linked with a move to Ferrari, and Horner presumed Vettel was joining the Italian team.

"Obviously Ferrari have made him a very attractive offer," Horner said on Saturday.

"The lure of Ferrari; a window has opened there with whatever is going on and he has decided the timing is right for him."

Current Ferrari drivers Fernando Alonso and Kimi Raikkonen are both under contract with the Italian team for 2015, but Alonso acknowledged he had been in recent talks with the team about his future.

Asked directly whether he could be driving for either of rival teams McLaren or Red Bull next year, Alonso said: "This is a very difficult question to answer. And I will repeat the answer, probably I will do the best for Ferrari."

With the Red Bull option now closed, McLaren looms as the likely destination for Alonso should he leave Ferrari. McLaren will enter a new phase in 2015 with a switch to Honda engines as the Japanese automaker re-enters the sport.

Outgoing Ferrari president Luca di Montezemolo sounded resigned to losing Alonso when he spoke earlier this week at the Paris Motor Show.

"At this moment, in Japan, Alonso is discussing his future at Ferrari," di Montezemolo, still Ferrari's president until Sergio Marchionne takes over later this month, said.

"Whether he stays or doesn't stay, he has done a lot for Ferrari. We have a contract and in the next few days we are going to take a final decision with a lot of respect for one another.

"If he says he will stay, we will be happy. If we decide together to end our collaboration, it will be in the best manner possible."

Vettel joined Red Bull in 2009 and won four-straight championships between 2010 and 2013, although he has yet to win a race in 2014 when the Mercedes team has been dominant and he has been outperformed by young Australian teammate Ricciardo.

"We want to warmly thank Sebastian for the incredible role he has played at Infiniti Red Bull Racing for the last six years," Red Bull said in a statement.

"As we wish Sebastian well in the next stage of his career, we also look to the future with excitement, as the vacancy makes way for the next generation of Red Bull racers."