Sagan and Mark Cavendish (Dimension Data) collided and Cavendish crashed in the Tour's fourth stage on Tuesday in Vittel.
"There was a very long discussion, it's not an easy decision to take, it's not because it's about Sagan, but it's about the act that a rider made," UCI Jury president Philippe Marien said.
"We are at the beginning of the Tour de France. Before the Tour de France, we warned the sprinters that we would look very close at every sprint. And that is what we did today.
"It was not an easy decision, but it's the beginning of the Tour and this is now the moment to set our boundaries, and that is what we have done today."
The Tour press release from the UCI jury referred to article 12.1.040/10.2.2 Sprint Irréguilier. Sagan went from second place to taking the car home.
Cavendish finished the stage but had to go to the hospital for check-ups. His Dimension Data team fears a problem with his right shoulder.
"The jury decided to disqualify Peter Sagan because of the very serious manoeuvre in the sprint. I didn't know anything about the first crash [at 1.5km out], it's only about this crash," Marien continued.
"This article 12.1.040 says that the jury can decide to send a rider home after a very serious violation.
"Because of the fact that we decided we can apply this article because it's a very severe violation, it doesn't matter if it's the name Sagan, this disqualification is qualified and justified."
Sagan appeared to close to the right and leave no room for Cavendish and the world champion's elbow came out to the right when doing so.
"In every sprint something happens, but what happens there, it looks like it was on purpose and it almost looks like hitting a person," he added.
"It's not about Sagan or Cavendish, it could be Joe Blow. It doesn't mater, we just take our action."
The jury looked also looked at an incident between eventual stage winner Arnaud Démare (FDJ) and Nacer Bouhanni (Cofidis).
"But that looked to me to be much less severe than the Sagan and Cavendish incident."
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