Lewis Hamilton's worst fears have been realised, as he began his world championship defence down among the tailenders.

He was well off the pace in opening practice for the Australian Grand Prix in Melbourne which was dominated by Williams, Brawn GP and Toyota, the controversial "Diffuser Three".

For Hamilton, who is accustomed to competing at the front of the grid, it was a humbling experience, setting only the 16th fastest time in the morning.

It got worse in the afternoon when he slumped to 18th out of 20 starters. With a best time of 1min 27.813sec, Hamilton was nearly 1.8sec slower than Nico Rosberg of Germany, who led both sessions in his Williams.

In Formula One terms, that is a massive margin which, if carried into the race, would result in Hamilton suffering the embarrassment of being lapped.

However, the British driver tried to put a brave face on his troubles and said: "My day was not too bad. We're not as quick as we would love to be, but we are working very hard. We got through quite a programme and made some decent steps for us. We're happy with what we have done today."

Attempting to explain why the McLaren was so slow, Hamilton said: "It's just a general lack of grip everywhere, it is not one particular area of the corners. We cannot carry the speed through corners we would like. But there is no quick fix, it is going to take time."

Hamilton and team-mate Heikki Kovalainen were even slower than Force India's Adrian Sutil and Giancarlo Fisichella, consistent tailenders last season but whose cars are now powered by the same Mercedes engine as that fitted to the McLaren.

Hamilton had gone into the opening round of the 2009 title chase reluctantly accepting that his new car was so uncompetitive that he was unlikely to achieve the top-eight finish in Sunday's race that would give him at least a point. But the 24-year-old, who won last year's race from pole position, could not totally disguise his dismay at just how far off the pace his car was.

At one stage towards the end of the afternoon session he was heard complaining over his cockpit-to-pits radio: “The car is bottoming so much I can hardly see the braking points.”

Hamilton was beaten by team-mate Kovalainen, although the Finn was only just faster in 17th place in the second session. Ferrari also struggled and finished with Felipe Massa 10th and Kim Raikkonen, 11th.

At the front, the six cars at the centre of the diffuser row — belonging to Williams, Brawn and Toyota — were in the top seven in the faster afternoon session.

Rosberg, son of former world champion Keke, stopped the clock on 1min 26.053sec. He was followed by Rubens Barrichello of the Brawn team, who today were set to announce a multi-million pound sponsorship deal with Richard Branson's Virgin group, and the Toyota of Jarno Trulli.

Australian Mark Webber, the Red Bull driver still recovering from a broken leg sustained in a cycle race, delighted the home crowd by steering the first of the conventional' cars to fourth place ahead of Britain's Jenson Button in the second Brawn, Timo Glock (Toyota) and Kazuki Nakajima (Williams).

Doubt still surrounds the legality of the Williams, Brawn and Toyota cars even though they cleared a technical check yesterday.

Rivals claim those teams are gaining a significant advantage from the design of the diffuser that is said to contravene new rules aimed at producing closer racing.

Ferrari, Renault, Red Bull and BMW Sauber have appealed against the decision, and the FIA will rule on the matter after the Malaysian Grand Prix on Sunday week.