Chris Boardman struck his head against a post or stone wall but apparently escaped serious injury. (Patrick Kovarik/AFP)

The British rider sustained a deep cut above one eye and was carried by stretcher into an ambulance. He was taken to a hospital for a precautionary brain scan.

Tour director Jean-Marie Leblanc said Boardman fell after the front wheel of his bike touched the back wheel of his teammate, Frederic Moncassin.

Leblanc said Boardman struck his head against a post or stone wall by the roadside, but that he had been protected by his helmet. The cut above the eye apparently caused no serious damage, Leblanc said.

The crash occurred about 90 miles into the 127.4-mile race outside the town of Youghal.

The withdrawal made Boardman ineligible for the rest of the tour and ended his goal of retaining the yellow jersey into the first stages in France.

Boardman won the 1992 Olympics pursuit gold, and won the prologue in his tour debut in 1994. But he smashed an ankle in the 1995 prologue, finished 39th in the 1996 Tour, and in 1997 again won the prologue but was forced to withdraw at stage 13 because of crash injuries.

He won this year's Dublin prologue, and retained the yellow jersey Sunday when his main rival, Mario Cippolini of Italy, was involved in a less serious crash at the start of the final sprint.