The 36-year-old, who has competed in seven Tours de France and was regarded as the world's best lead-out rider, was involved in a pile up only 5km into the 210km stage in the Pyrenees.
Organisers cut short the stage before the final two climbs of the day as snow, wind and rain hit the peloton. A number of riders pulled out of the tour on a challenging day, including RadioShack-Nissan's Andy Schleck and Movistar's Alejandro Valverde who did not even start the day, but Dean's accident overshadowed events.
Dean won a bronze medal on the track at the 1994 Commonwealth Games and also competed at Atlanta in the team's pursuit but switched to the road for the 2000, 2004 and 2008 Olympics.
He had been expected to team up with Greg Henderson for the road race at London but his probable unavailability might open the door for someone like Jack Bauer or allow BikeNZ to enter one of the track riders to beef up their track team.
Fellow Kiwi SamBewley tweeted: "Heart sunk as I just read a tweet that Julian Dean has broken his leg. Too many bad things happen to too many good guys! All the best Jules!''
- Michael Brown of APNZ
