Sagan did go to the Dimension Data bus after the race to speak to Cavendish, who crashed out of the 2014 Tour in the first stage after breaking his collarbone in a sprint finish fall he later admitted was his fault.
Arnaud Demare became the first French stage victor of this year's Tour, crossing the line first in Vittel ahead of Sagan and Norway's Alexander Kristoff, but his achievement was overshadowed by the two serious crashes on the final sprint.
Race jury president Philippe Marien of the UCI says, "We have made a decision to disqualify Peter Sagan from the 2017 Tour de France after the tumultuous sprint here in Vittel, where he endangered several riders, including Mark Cavendish and others who involved in the crash".
"[I was] following Démare around and just Sagan came over", said Cavendish.
"We apply article 12.104 of the International Cycling Union regulations which allow us to disqualify a rider".
I don't know if I've snapped the fake ligament in or what - I'm going to go and get it looked at.
"A crash is a crash, I'd just like to know about the elbow really".
Cavendish collided into the barriers with his right shoulder and hit the deck at top speed.
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Initial reports said Sagan had been penalised 30 seconds and 80 green jersey points, although that has now been changed to a disqualification.
Cavendish went to hospital with his right arm in a sling and his Dimension Data team said that it is unsure he can continue in the Tour de France on stage five.
However, Bora-Hansgrohe's Sagan took a second to clip his shoe back in before accelerating to outsprint Australian Michael Matthews of Team Sunweb and Ireland's Dan Martin of Quick-Step Floors. "Happy to get through unscathed", Thomas said.
Yellow jersey holder Geraint Thomas crashed earlier in the finale but since the incident occurred within the final three kilometres, he will be credited with the same time as the victor.
"It's wonderful, beating all the best sprinters like that at the Tour de France is something I'd hoped for, for a long time", said Demare. He initially lost all the points he earned on Tuesday and fell far back in the race for the green jersey, a punishment that seemed fair given his excessive use of elbows.
His team leader and reigning champion Chris Froome was a place behind in ninth and moved up from sixth to second overall, still at 12sec. "There was a crash in front of us and again I had nowhere to go".
Slovakian cyclist Peter Sagan.
The 160.5-kilometer (100-mile) leg begins in Vittel and winds its way to La Planche Des Belles Filles with a short but steep finishing ascent that features a leg-breaking 20-percent gradient in the final meters.