Horner suspects Ricciardo's decision was linked to Verstappen

Who could replace Ricciardo at Red Bull? (2:06)

As Daniel Ricciardo announces a switch to Renault for 2019, Laurence Edmondson wonders who might take his Red Bull spot. (2:06)

Red Bull team boss Christian Horner believes Daniel Ricciardo's decision to switch to Renault next year was partly influenced by the growing strength of current teammate Max Verstappen.

Ricciardo announced last week that he would leave Red Bull and drive for Renault next year after signing a two-year contract with the French manufacturer. In terms of outright competitiveness, the move represents a step backwards for Ricciardo, with Renault unlikely to eliminate the significant gap in performance to Red Bull in the short-term.

However, Ricciardo said he wanted "a fresh and new challenge" next year, and emphasised the impressive record of Renualt in F1 over the history of the sport. Yet Horner suspects that the prospect of going up against current teammate Verstappen as he becomes a stronger driver also played a part.

"Daniel said he decided after a long flight to America that he wanted a change," Horner told F1's official podcast Beyond the Grid. "If you look at the rational reasons for that, it's difficult to understand but Daniel obviously had his reasons.

"I think it came down to wanting to take on, in his words, 'a new challenge' but I also think that he sees Max growing and growing in terms of speed and strength and he doesn't want to play a support role -- for want of a better word because it's not that they are treated in any way different.

"[They would have been guaranteed] absolute equal status as they have always had [at Red Bull], but I can't help but feel that was a large part of Daniel's decision making. I could understand if it was to Ferrari or Mercedes, but it's an enormous risk at this stage of his career."

Horner revealed that Ricciardo only let him know about the decision the day before it was announced and said he first thought his driver was winding him up. However, he admitted that the signs had been there during their own negotiations with Ricciardo and that the Australian had been reluctant to commit to Red Bull.

"I have to admit, it's been a bit like trying to convince a girl to go out with you who's being pretty reticent," he said. "I guess when you look at it it's felt like that.

"Daniel's had conversations with Dietrich, with myself, with Helmut and we have bent over backwards to make it happen, but if your heart is not really in it then ... it's just felt like that.

"At the end of the day, we have given Daniel everything that he ever wanted and it still wasn't enough in his mind to say 'I want to keep going at Red Bull'. So it wasn't about money, it wasn't about status, it wasn't about position or commitment or duration, I think he felt that he needed to take something else on at this stage in his career.

"It might be an inspired choice, it might be one that he regrets."