McLaren have indicated they are likely to drop Jenson Button and replace him with Stoffel Vandoorne next season.
Button, 36, is out of contract at the end of the year, while Vandoorne, McLaren's reserve driver, is one of the hottest properties without a race seat.
McLaren chairman Ron Dennis said the 24-year-old Belgian was "an integral part of McLaren-Honda's future".
He added: "Any team that imagines they may be able to poach him is very much mistaken."
Two-time world champion Fernando Alonso is contracted to McLaren to the end of 2017 so Button and Vandoorne are essentially in competition for the remaining seat.
Dennis said McLaren were "not yet ready to commit or communicate" on their 2017 line-up, but added: "You may rest assured that Stoffel is not for sale."
A man in high demand
Reigning GP2 champion Vandoorne deputised for the injured Alonso at last month's Bahrain Grand Prix, out-qualifying Button before finishing 10th on his grand prix debut.
Dennis added: "Stoffel is an extremely talented, intelligent and hard-working young driver, with an enormous amount of potential and who, importantly, knows how to win."
Renault are known to be interested in Vandoorne for next season.
Their racing director Frederic Vasseur ran him in his teams in the GP2 and Formula Renault 3.5 feeder categories and rates him highly.
But McLaren have Vandoorne under a contract this year that prevents him signing for another team until a specific date in the autumn, believed to be around the end of September or beginning of October.
Where does this leave Button?
Button does have other options to extend his F1 career into an 18th season should McLaren decide not to keep him.
Williams are known to be interested in him as a potential replacement for Brazilian Felipe Massa, who is likely to be dropped at the end of the season.
Finn Valtteri Bottas is said by insiders to be under contract for another season.
There are also seats potentially available at both Mercedes and Ferrari, where Nico Rosberg and Kimi Raikkonen are out of contract at the end of the season.
However, Mercedes are expected to agree a deal for championship leader Rosberg to stay within the next few weeks and would anyway be unlikely to sign a driver who turns 37 next January.
Ferrari will not make a decision on their 2017 partner for Sebastian Vettel until August but are likely to retain Raikkonen for at least another season.