London - Jenson Button's Formula One future is looking increasingly uncertain after McLaren boss Ron Dennis said on Thursday he was keeping an eye on other leading drivers who might become available.
Button, the 2009 world champion with Brawn, has been at McLaren since 2010 and is in the final year of his current contract.
The 34-year-old Briton, winner of 15 grands prix, is the most experienced driver in Formula One, having made his debut in 2000, and is currently eighth in the championship with 68 points.
Danish rookie team mate Kevin Magnussen has scored only 37 but has outqualified Button repeatedly and, at 21, is seen as a face of the future.
Dennis told the official formula1.com website that he was satisfied with the current line-up, and both were capable of winning in a competitive car - which McLaren have been unable to provide this year.
Despite that, he said they needed to “be responsible enough to appreciate that McLaren will always make efforts to hire the best drivers available.
“Jenson and Kevin represent an excellent blend of capable experience and youthful promise, and we're very happy with both of them,” he added.
“The fact that we're keeping an eye on what a few other drivers are up to in no way contradicts that, because, as I say, if opportunities arise, we'll appraise them - we always have and we always will.”
Media reports have repeatedly linked McLaren to Ferrari's Fernando Alonso, who had an unhappy season at the team alongside Lewis Hamilton in 2007 and fell out with Dennis.
The Spanish double world champion has a contract at Ferrari until the end of 2016 but there are likely to be performance clauses in the small print.
Ferrari, who have not won a drivers' title since Kimi Raikkonen in 2007, are currently third in the constructors' standings and only 10 points clear of resurgent Williams in fourth place.
McLaren have not won a race since 2012 and failed to get on the podium entirely last year.
However, Honda are returning to the sport as McLaren's engine partners next year and, while Button has close ties to the manufacturer and to Japan, are said to be pushing for an “A List” driver.
Asked whether McLaren would be prepared to “move mountains” to get their man, whoever he might be, Dennis replied: “Well, that depends on whether you respect drivers' contracts or not, and I do.”
Button recognised after Sunday's Belgian Grand Prix that his future was up in the air.
“If I have to retire at the end of the season then so be it, but I feel I have so much more to give and I can't imagine life without motorsport and especially Formula One,” he told the BBC.
McLaren racing director Eric Boullier was non-committal about 2015 and Button's place in the team's plans.
“We have discussions open with him, but until we have decided on our strategy, we will not finalise our driver line-up,” said the Frenchman.