Qualifying times

The Williams driver posted a time of one minute 12.275 seconds to edge out BAR's Button by less than one tenth of a second.

Jarno Trulli will start in third with Juan Pablo Montoya in fourth.

World championship leader Michael Schumacher was more than a second off the pace and starts sixth on the grid.

It was the worst qualifying session of the season for the world champion, who is hoping to become the first driver to win the same Grand Prix seven times.

"It's true it's my worst qualifying position but I'm still sixth and I've won races from worse positions than that," he said.

I'm a little bit disappointed but it's still good to be on the front row Jenson Button

"I said we would have a fight this weekend but I think we have a better competitive package for the race compared to qualifying.

"I made a little mistake on my lap but not enough to explain the difference."

Button admitted he thought he had pole in the bag.

"Before Ralf went out I was excited because I was up on everybody else by seven tenths of a second, so I'm a little bit disappointed.

"But it's still good to be on the front row," he said.

"I'm sure the engine has helped us in qualifying, not just in power but reliability. Hopefully it will help us in the race where we've a good strategy."

Ralf's pole is the fifth of his career and Williams' first since Colombian Juan Pablo Montoya achieved the feat at the German Grand Prix last August.

But Ralf, whose best finish this season was fourth in the opening race of the season in Australia, is cautious of how he will perform on Sunday.

The 28-year-old, who has failed to finish in the last two races, said: "Being on pole you are always confident you will have a good race.

"But we have clearly been struggling for the last three or four races, so it's going to be a tough one, and with Jenson on my tail it's going to be even tougher."

Japan's Takuma Sato was quicker than BAR team-mate Button through the first timed sector but blew his chances of a second front-row start in succession with a 360-degree spin at the final corner.

Sato was lucky not to hit the concrete wall but his mistake left him well down the grid and he will start from 17th position.