he outcome of the 2007 drivers' world championship has been plunged into renewed doubt after McLaren declared its intention to appeal the decision of the Interlagos stewards not to penalise Williams and BMW Sauber for alleged fuel irregularities.

After Sunday's Brazilian Grand Prix the stewards analysed fuel samples taken from the Williams and BMW cars during their pit stops, which appeared to show temperatures outside the permitted range.

The enquiry into an apparently arcane technical issue had potentially huge repercussions, since Williams driver Nico Rosberg and BMW's Robert Kubica and Nick Heidfeld finished the race fourth, fifth and sixth respectively - ahead of Lewis Hamilton, who lost the world title to Kimi Raikkonen by a single point after a disastrous race.

Had the three drivers been disqualified, Hamilton would have been promoted to fourth place and declared world champion.

But because the stewards deemed the evidence inconclusive, they opted not to impose any penalties and upheld the original result, confirming Raikkonen as champion.

However, a McLaren spokeswoman told ITV-F1.com late on Sunday evening that the team intends to challenge the stewards' ruling in the FIA's International Court of Appeal - leaving the title result hanging in the balance once again.

McLaren has notified the governing body of its intentions but no appeal date has yet been set.

A couple of hours earlier, in a statement explaining their decision, the stewards said they could not prove that the fuel used by Williams and BMW was illegal.

Article 6.5.5 of the Formula 1 technical regulations states: "No fuel on board the car may be more than 10 degrees centigrade below ambient temperature."

FIA technical delegate Jo Bauer reported that the fuel samples from all four cars showed temperatures 12-14C lower than the ambient at the time.

But the stewards said they could not be certain the temperatures were outside the 10C limit due to conflicting evidence.

They pointed to a discrepancy between the ambient temperature recorded on the Formula One Management timing monitors and that provided by the FIA and team-contracted meteorologists Meteo France - and said there was no "regulation stating in clear terms that for the purposes of Article 6.5.5 the definitive ambient temperature shall be indicated on the FOM timing monitors alone".

They also said they lacked "a precise reading of the temperature of 'fuel on board the car' which shows fuel at more than 10 degrees centigrade below ambient temperature".

Their statement concluded: "In view of the matters referred to above, the stewards consider that...there must be sufficient doubt as to both the temperature of the fuel actually 'on board the car' and also as to the true ambient temperature as to render it inappropriate to impose a penalty".