Sebastian Vettel not yet a great - Sir Jackie Stewart

Former F1 champion Sir Jackie Stewart does not believe Sebastian Vettel can yet be called one of the greats, even if he wins a third straight title.

The Red Bull driver won his fourth race of 2012 in Korea last weekend to take the lead in the drivers' standings.

But Stewart told BBC Sport: "If you've got a superior car, it's relatively easy to win the championship.

"To really show you're one of the greatest, you have to deliver when you haven't got the best machinery."

The three-time world champion added: "I have the highest respect for Sebastian, but he could not have achieved what he has without the [Red Bull design chief] Adrian Newey factor."

Vettel won his maiden F1 title in 2010 in what was only his third full season in Formula 1. The German defended that championship in dominant fashion last season, winning 11 of the 19 races.

This season Red Bull initially struggled for pace and reliability and up until the 14th grand prix Vettel had won just one race - round four in Bahrain - to trail championship leader Fernando Alonso of Ferrari by 39 points.

But Red Bull then improved their car and the 25-year-old has since become unbeatable, winning in Singapore, Japan and Korea to take a six point lead from Alonso with four races to go.

"I'm a big Sebastian Vettel fan," added Stewart. "He completely dominated the Korean Grand Prix and drove in such a mature fashion for a 25-year-old. It was more than impressive.

"Vettel has established himself as the favourite and Alonso is now on the back foot.

"Led by Adrian Newey, Red Bull have bounced back from a slow start to the season and have now come up with superior equipment."

On Monday, BBC Sport linked Vettel - who is two wins away from equalling Stewart's record of 27 career victories - with a move to Ferrari in 2014.

But Stewart said: "I think that's premature. He would be mad to leave Red Bull at the present time.

"I know there would be a difference in their basic salary, but he's in a position where he's winning grands prix.

"He's won 25 races and is going to knock me off my perch before the end of the season - but he's only 25 and still has time to move."