Keen observers were squinting at the sky for signs of rain clouds on Sunday morning as the paddock tension rose ahead of the title-deciding Brazilian Grand Prix.

Amid a hubbub that resembled the atmosphere of a football cup final, the two title contenders Briton Lewis Hamilton and his nearest rival and home hero Brazilian Felipe Massa did their best to remain calm and composed.

Hamilton leads Massa by seven points and needs only to finish fifth or better to become the youngest and first black champion in Formula One history.

He is also seeking to end Ferrari`s grip on the title by succeeding outgoing champion Finn Kimi Raikkonen and bring the title home to McLaren Mercedes for the first time since another Finn Mika Hakkinen won the title for them in 1999.

But despite his lead and his comfortable position of fourth on the grid - with Massa on pole and his team-mate Raikkonen third, the pair split by Italian veteran Jarno Trulli`s Toyota - Hamilton knows that just like last year anything can happen.

Twelve months ago in the 2007 showdown, when he led then McLaren team-mate two-times champion Spaniard Fernando Alonso by four points, Hamilton allowed his nerves to beat him.

He made mistakes on the opening lap, dropped down to seventh from second on the grid and then suffered gearbox problems on his way to finishing a distant, and controversial, seventh.

Raikkonen, who won the race, took his first championship after starting the race seven points behind the 23-year-old English rookie.

And that is why the weather is so important.

In hot and dry conditions, Ferrari have this season performed at their best. In the wet, or cold, McLaren have been dominant.

In changeable, mild conditions, the two teams have often been so keenly matched that it has been difficult to separate them.

Rain, and some of it heavy, was forecast for Sunday - thus giving Hamilton hope that he can keep Massa in his sights, but like all of his rivals, knowing also that he has to avoid mechanical problems and finish the race with some points.

After last year`s misery, Hamilton knows the pitfalls well as he attempts to become the first British champion since Damon Hill in 1996 and he has the support of a solid McLaren team and the British prime minister behind him.

On Saturday, Gordon Brown said: ``Lewis Hamilton is a hero to so many young people in Britain. I think he will be a great world champion. He just missed out last year. I think everybody will be rooting for him.`

McLaren team chief Ron Dennis said: `We have done everything we can to eliminate the problem of a mechanical failure. We have run every single possible scenario that we could think of - and we introduced all the problems that we imagined could occur and factored those in.

`Can we damage a nose? Can we stop? Can we change a nose and still come fifth? All those things we factored into every scenario we could think of.

`We increased the intensity of the real-time analysis so we should be able to pick up any variation or tyre pressure within two or three seconds so that if he does pick up a puncture and he`s a few hundred yards from the pit-lane entrance that we can pick it up fast enough.

`Every single part on the car has been re-analysed. We looked at the history over the last year of each component`s reliability, anything that falls close to being of concern.

`I can say whatever the outcome, if our car fails - and we don`t want that to happen - that no more effort could have been put in than we put in.`

Ferrari, just as intensively, have prepared their cars for Massa and Raikkonen, but with a more laid-back attitude. They know that they need to target a one-two finish, with Massa first, to maximise their chances of glory, but admit that even if they do that and succeed Hamilton can be champion by finishing third, fourth or fifth.

`We are not thinking of Lewis, just ourselves and what we have to do,` said Massa. `If we think of Hamilton all the time we would go crazy!`

Mixed into the possibilities, too, is the prospect of another driver winning or interfering with the duel for the crown - notably Trulli, or even Alonso, who is starting right behind him on the third row of the grid.

And, on a more sentimental note, 37-year-old Scot David Coulthard starts his 246th and final race before retiring from Formula One after 15 years with Williams, McLaren and Red Bull.

Around ten years ago, Coulthard was involved in McLaren`s young driver programme and put in some time giving guidance to the talented hopefuls on the team`s books.

One of those was a 13-year-old boy from Stevenage who told him that his ambition was to get to Formula One and win the world championship, like his hero Ayrton Senna.

By a wry and unhappy coincidence, Coulthard had entered F1 as the successor to Senna at Williams, following the great Brazilian`s death in 1994. Senna, in his own way, had been an inspiration to the Scot as well.

The boy, of course, was Hamilton who Sunday afternoon seeks to achieve his dream at the expense of Massa. And if he fails, again, then Massa will be the first Brazilian champion since Senna last took the drivers` crown in 1991.