(REUTERS) - Yamaha's MotoGP championship leader Valentino Rossi will start last in Sunday's title decider in Valencia after a Court of Arbitration for Sport ruling on Thursday, leaving his hopes of a 10th world title hanging by a thread.
The Italian had appealed for his back-of-the-grid penalty to be suspended to allow him to race on equal terms with Spanish title rival and team mate Jorge Lorenzo pending a further appeal.
However, MotoGP said in a statement that the Lausanne-based court had issued an order dismissing Rossi's application for a stay of execution.
"As a consequence of this ruling, at the Gran Premi de la Comunitat Valenciana, last race of the FIM world championship grand prix, Mr Rossi will start from the last position on the grid," it added.
The 36-year-old has won seven of his titles in the top category and is battling against Yamaha team mate Jorge Lorenzo for another.
The CAS added, though, that the arbitration procedure remained in progress and a final decision had yet to be made on the merits of the case.
Rossi leads Spanish rival Lorenzo by seven points, with 25 for a win, ahead of a finale that has been overshadowed by controversy since the last round in Malaysia.
The row kicked off, literally according to Rossi's accusers, when the Italian tangled with Honda's outgoing double world champion Marc Marquez at Sepang.
The young Spaniard has said Rossi used his leg to make him crash, an allegation the Italian has denied while suggesting Marquez was trying to sabotage his championship bid and help Lorenzo.
Race stewards allowed Rossi to keep his third-place result in Malaysia but handed him three penalty points which, added to an earlier one, triggered an automatic demotion for Valencia.
The sanction has divided the sport, fuelled national rivalries and led to finger-pointing in both directions.
Italian Prime Minister Matteo Renzi called Rossi to express his support while Spanish counterpart Mariano Rajoy took to Twitter to back Marquez. Yamaha have taken up the cudgels against Honda.
The Spanish media have compared Rossi's actions to those of Italian defender Mauro Tassotti, who elbowed Spain's Luis Enrique in the face in an infamous incident in the quarter-finals of the 1994 football World Cup.
On the other side, Marquez filed a legal complaint after he and his family were allegedly insulted and physically attacked by Italian television reporters at their home near Barcelona.
Hundreds of thousands have also signed an online petition calling on MotoGP organisers to drop the Valencia penalty.
Such has been the atmosphere that the International Motorcycling Federation (FIM) and promoters Dorna cancelled Thursday's pre-race news conference and summoned riders "to address the situation".
"The recent events... have had a damaging effect on the staging of our competitions and poisoned the atmosphere around the sport," said FIM president Vito Ippolito. "I express the hope that ... in Valencia the riders will fight it out on the track and in a way that fully respects the spirit of fair play."