Christian Horner thinks Sebastian Vettel will force a move to Mercedes for the 2018 season if Ferrari does not prove it can challenge for championships.
Vettel and Ferrari have failed to win a race in 2016 despite high expectations of challenging Mercedes for the title ahead of the season. The German driver left Horner's Red Bull in 2014 after four consecutive world championships.
Despite Vettel's three wins last season, Ferrari has failed to build on its winter optimism and has recently suffered a spate of gearbox issues, resulting in back-to-back grid penalties for Vettel. Ferrari team boss Maurizio Arrivabene has said the team "cannot fail" coming to Hungary this weekend, with the outfit under increasing pressure from president Sergio Marchionne.
Horner does not think Vettel, who has dropped to fifth in the drivers' championship, will be happy staying in an environment where he cannot perform at his best.
"I can well imagine that in 2018 Seb will switch to Mercedes if he doesn't start making progress with Ferrari," Horner told Bild. "I know from personal experience that he has to feel good in order to perform well. And I'm not sure whether he likes Sergio Marchionne's pressure."
A Vettel move to Mercedes would set up a mouth-watering partnership with Lewis Hamilton, who is contracted until the end of 2018. Championship leader Nico Rosberg is yet to sign a contract extension beyond 2016 but is widely expected to do so.