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The Texan claimed his third stage win of this year's race to extend his overall lead over Italian Ivan Basso to three minutes and 48 seconds.

Setting off last after claiming the yellow jersey on Tuesday, Armstrong covered the 15.5km in 39:41.47.

He finished over a minute clear of Jan Ullrich to move closer to victory.

Ullrich showed his determination with a strong effort, but he was still unable to match his great rival's relentless rhythm.

Starting the day a minute-and-25 seconds ahead of Basso, Armstrong had taken over a minute out of his nearest rival by the 9.5km mark.

He was also 40 seconds quicker than Ullrich at that stage, and his relentless charge continued over the final 6km.

The 32-year-old passed Basso, who started two minutes ahead, with several kilometres remaining and became the only rider to clock under 40 minutes.

I had to be good because I knew all the others would be good Lance Armstrong

Armstrong sprinted to the finish on the historic stage, the first time a time trial had been held on traditionally the toughest climb of the race.

Despite his delight at winning the stage, the American criticised the decision to hold the time trial on a mountain route thronged with up to a million spectators.

"Although I enjoyed my day, I think it is a really bad idea to do a time trial up this mountain," Armstrong said.

"With the whole history of it, there is too much at stake, and a million people up the sides makes it very dangerous.

History beckons Armstrong

"Some of them have been up there for a week drinking and I was lucky I made it through some of those sections."

Armstrong is now well placed to win his sixth successive Tour, with only one big mountain stage left on Thursday before three more on the road to Paris.

"It is still a tight race," he added. "I had to be good because I knew all the others would be good.

"All in all, it was a very important day."