LONDON, Oct 12 (Reuters) - Speculation about Fernando Alonso's Formula One future is shifting into overdrive as the champion prepares to fight for his third title in a row in Brazil next week.

The season-ender at Interlagos could be the unhappy Spaniard's last appearance for McLaren, just 17 races after he
made his debut with the Mercedes-powered team in March.

The 26-year-old, winner of the past two championships with Renault, is increasingly at odds with a team he has accused of favouring British rookie team mate and championship leader Lewis Hamilton.

That estrangement was highlighted by the governing International Automobile Federation (FIA) deciding this week to appoint a special scrutineer to ensure equal treatment in Brazil.

Four points behind 22-year-old Hamilton, Alonso could yet become the first driver to win back-to back titles with
different teams since the late Argentine Juan Manuel Fangio in 1957.

His future beyond that is more complicated. The Spaniard is contracted for at least another year with McLaren, who have put talks on hold until the season is over.

"There is at this stage nobody engaged in any dialogue whatsoever," McLaren boss Ron Dennis said at last month's
Japanese Grand Prix. "What you are looking at is speculation and people who want to contribute to any process that destabilises the team or de-focuses the drivers."

Alonso and Dennis have been barely on speaking terms since August, with the Spaniard also contributing evidence to a spy hearing that fined McLaren $100 million and stripped them of their constructors' points, but there will be plenty to talk about after Interlagos.

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Austrian Niki Lauda, who won his third title with McLaren, told Reuters last weekend that McLaren could play hardball with the driver.

"Ron is not an easy man but I always knew the limit, I cannot damage my sponsors or the people who pay my money," he said. "It will be interesting to see what happens after Brazil."

If Alonso leaves, Renault are seen as his favoured option with Toyota also mentioned but representing a riskier
alternative as a team who have yet to win anything despite vast resources.

Champions Ferrari have ruled out any change to their lineup for 2008 but the paddock rumour mill has refused to give up on a 'dream ticket' of Alonso paired with Finland's Kimi Raikkonen after that date.

Finnish tabloid Ilta-Sanomat reported on Thursday that it understood Alonso had agreed a preliminary contract with Ferrari president Luca di Montezemolo for three years from 2009.

However Germany's Bild newspaper maintained that the Spaniard had agreed a three-year deal with Renault, who refused to give any credence to the report.

"The team will make no comment on any media speculation of this kind. When we are ready to announce our 2008 driver line-up, we will do so," said a Renault spokesman.