Jenson Button was today warned his world championship challenge faces a two-pronged attack from Red Bull drivers Sebastian Vettel and Mark Webber.

Red Bull boss Christian Horner insisted there would be no team orders as Vettel and Webber seek to cut into Button's championship lead in this weekend's Italian Grand Prix at Monza.

As reigning title holder Lewis Hamilton set the fastest time in the opening practice session in a McLaren that was able to exploit it's power-boosting KERS system on the high-speed circuit, Horner claimed it was only a matter of time before Button was hauled in.

With five rounds of the title chase to come, the British hope, winner of six of the season's first seven Grands Prix, holds a 16-point advantage over Brawn GP team-mate Rubens Barrichello, with Vettel and Webber respectively 19 and 20.5 points behind.

But Button's form has faltered just recently and Horner said: "Before long somebody is going to take a big chunk out of Jenson. He has got away pretty lightly so far.

"I think he has only scored 11 points since Silverstone in June. Both of our boys have scored 24 and had the potential to score a lot more than that.

"For us, it doesn't make any sense to back one over the other. They're still very tightly matched and it is impossible to predict which one will emerge at the top. We'll continue to back both equally and may the best man win."

If Button is feeling the pressure, he didn't show it this morning at the Monza Autodrome.

He finished seventh fastest 0.770secs slower than Hamilton but significantly faster than Australian Webber, who was ninth, and Vettel, who was 18th after completing only eight laps as he sought to conserve his engine supply.

The young German sensation, winner in Italy last September for the Toro Rosso team, has suffered four major engine failures this season. He now faces the prospect of using more than the permitted maximum of eight engines before the end of the campaign, and having to take a race-wrecking grid penalty as a consequence.

Horner also predicted the KERS cars of McLaren, Ferrari and Renault would prove impossible to overtake.

KERS (Kinetic Energy Recovery System) was introduced this season as a means to demonstrate that Formula One can help develop green racing technologies.

It is a means of storing energy that would otherwise be wasted under heavy braking. A driver can then press a button on his steering wheel to release this energy as a power boost on the straights - and the Italian circuit is famous for it's long straights.

The drawback to KERS is it's huge cost, and only three teams will use it at Monza. Asked what chance a non-KERS car like the Red Bull or Brawn GP had of overtaking a KERS car, he replied: "I would say there is Bob Hope and no hope of that, to be honest.

"They have effectively another 80 horsepower in a straight line. As soon as they hit their magic button it is impossible to get anywhere near them."

Giancarlo Fisichella, making his debut for Ferrari as replacement for injured Felipe Massa, was an encouraging eighth fastest.