Red Bull principal Christian Horner has claimed that the pace of Daniel Ricciardo was "probably a factor" in Sebastian Vettel's decision to join Ferrari.

The German, who won four world titles during his time at the Milton Keynes outfit, was regularly outshone by the Australian and finished 71 points behind his teammate in the 2014 standings.

However, Horner believes that Ricciardo's pass on Vettel at the Italian Grand Prix had an effect on his decision to leave Red Bull.

He told BBC Sport: "He [Vettel] was enormously frustrated after that Grand Prix.

"It was at a time where after the summer break, knowing Sebastian as well as I do, I could see he was very distracted and it was obvious something was at the back of his mind.

"And I think Ferrari were courting him quite hard and around Spa/Monza was the time he made the decision to do something different next year."

Vettel was replaced at Red Bull by Toro Rosso youngster Daniil Kvyat.