SINGAPORE (Reuters) - Mark Webber could have stayed in Formula One and challenged for grand prix victories for several more years had he not "fallen out of love" with the sport, according to Red Bull team principal Christian Horner.

Australian Webber, 37, announced in June he was leaving the team at the end of the season to join Porsche in their Le Mans sportscar programme.

"Mark's chosen to leave and only he knows the full reasons behind that," Horner said ahead of Sunday's Singapore Grand Prix. "In terms of his performance and capability, I think he could have done another couple of years, but I think he's fallen out of love a little with Formula One."

Webber, winner of nine races in seven seasons with Red Bull, has been eclipsed by triple champion team mate Sebastian Vettel since the German took his first title in 2010 and their relationship has been frosty.