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Red Bull team principal Christian Horner believes it is 'too early' for Jean-Eric Vergne to join the reigning world champions.
Vergne, along with team-mate Daniel Ricciardo and Lotus's Kimi Raikkonen, is in contention for the vacant seat alongside Sebastian Vettel next season.
Vergne is ahead Ricciardo in the championship but in terms of development, Horner believes Vergne is still behind Ricciardo.
"I think at this stage it's probably too early for him, which is why we chose to look at Daniel in the test at Silverstone," Horner said ahead of the Hungarian Grand Prix.
"I think [Vergne] deserves a seat in Formula 1. Should he not be successful with the Red Bull seat then it makes logical sense for him to continue at Toro Rosso. He's six months behind Daniel in terms of Formula 1 experience, so he's still learning and developing."
Horner went on to effectively rule Vergne out of contention.
"Kimi's qualities speak for themselves. He has a proven track record so I don't think you can question Kimi's credentials," he said.
"Obviously with Daniel, he's at the beginning of his career, for sure he has the speed and the rest you won't really know until you put himself into that situation."
"We've got a reasonable amount of information now to be able to make a decision. I think we want the two fastest and strongest drivers we can put in the car next year and I think both Kimi and Daniel would represent extremely good options."
Horner added that Red Bull will make the decision over the summer break and their choice will be announced "at Spa or Monza but it's not going to be later than that I don't think."
