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Christian Horner believes the lure of Ferrari is what convinced Sebastian Vettel to leave Red Bull and not the problems he has faced this season.

Vettel will leave Red Bull at the end of 2014 and has said he will announce his future plans very soon. With Fernando Alonso on the way out at Ferrari, Vettel is expected to replace him while Toro Rosso driver Daniil Kvyat will be promoted to Red Bull next year.

This season Vettel has struggled for results, with Renault's power unit failing to match that of Mercedes' and team-mate Daniel Ricciardo making the most of the package the team has got. What's more, design guru Adrian Newey is set to take a step back next year while aero boss Peter Prodromou is destined for McLaren.

Asked whether those factors convinced Vettel to leave, Horner said: "I don't think they were really factors. I think the lure of another team that's brightly coloured has come up and it's a point in his life and career that he's felt the timing is right for him.

"We obviously respect that decision and rather than looking to hold a driver against his will, if somebody's heart isn't here, it's better to reflect on the good times and look forward. I think he just fancied a new challenge.

"He's decided that his heart is somewhere else and we look back with a huge amount of pride about what we achieved together and it wouldn't be right to hold him back. If someone's heart isn't in the team it's better to release them. We now face a new era within the team."

Vettel told Horner he would be leaving on Friday night and the team wasted no time in announcing Kvyat as his replacement.

"Knowing Seb as well as I do, you could see in his body language that he has been a bit distracted. He came to see me last night and informed me of what his thinking and position was and from there we took action."

Asked if Alonso was ever an option, he said: "Fernando is a wonderful driver but our philosophy has always been to back youth and youngsters, with Sebastian, Daniel Ricciardo and now Daniil Kvyat. He looks a really exciting talent and prospect, so we're following our philosophy and I'm sure he's going to be a star of the future."

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