Lewis Hamilton's hopes of making a flying start to his defence of the world championship were rocked today when McLaren-Mercedes team chiefs made the shock admission that their new Formula One car lacked speed and was not capable of winning races.

With the opening grand prix in Australia just a fortnight away, Hamilton was nearly two seconds a lap slower than Rubens Barrichello's untried Brawn, powered by the same Mercedes engine as the McLaren, which set a blistering pace to top the times in yesterday's final round of testing at the Circuit de Catalunya near Barcelona.

Norbert Haug, vice-president of Mercedes motorsport who count McLaren as their principle team, said: "Basically, the car feels good, that is what our drivers say. However, we are not fast enough or competitive enough to aim for victories.

"We will continue to test next week at Jerez for another four days but it will take time to improve."

Martin Whitmarsh, McLaren's team principal, admitted the new MP4-24 car to be driven in the 2009 title chase by Hamilton and Finnish team-mate Heikki Kovalainen was performing below expectations.

He said: "It is a combination of factors. Our Mercedes-Benz engine is strong so MP4-24's performance shortfall is clearly chassis-centric.

"Inevitably, in 21st-Century Formula One, it is a car's aerodynamic aspect that confers the greatest pluses and minuses to its overall performance and that would appear to be the case with MP4-24."

But a bullish Whitmarsh went on: "F1 engineers can do great things when the pressure is on. Speaking to them today, I was struck by their resolute determination to address MP4-24's issues in record time.

"Many times in F1 history have successful teams started off with a car that was not working as well as they had hoped it would and many times have those teams engineered their way back to the front of the grid. That is what we aim to do."

By contrast, Ross Brawn, who bought the Honda team in a last- minute deal, could hardly contain his delight at the initial speed shown by Brazilian Barrichello and British team-mate Jenson Button.

It is difficult to gauge the true pace of cars in testing because they often run with different set-ups. But at the conclusion of the four-day session in Spain, Brawn said: "We are extremely pleased with the inaugural test.

"Our focus has been on reliability and achieving as much mileage as possible. Both have been achieved."