"We didn't expect him to be on the market this year, and our commitment is to our existing drivers. We think we've got a great pairing in Max and Alex, and think that has great potential for the future as well.
"Unfortunately, it's not something we are going to be able to accommodate."
Asked pointedly if that was 'a definite no then', Horner replied: "That is a definite no, I'm afraid. Sebastian is aware of that, so yeah, it's unusual for a four-time world champ to be out of a drive at this point in the year.
"I guess he's got a couple of choices available to him, I assume Racing Point, or a year out and re-evaluate the options in a year's time."
Given Sergio Perez is under contract for another two years, and brings with him considerable sponsorship from Mexico, and Lance Stroll has his father's backing behind him, it is hard to see Vettel joining a team that next year becomes Aston Martin.
Horner added: "I don't think Lance is going to get fired! Of course, driver's contracts within the teams, you never know what's within them.
"It's only the Contract Recognition Board that has full sight of them, so I'm sure that perhaps they have options or break points, or whatever, but that's really none of our business."
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