The Ferrari driver appeared to slow in the tunnel as Norris' rapid McLaren approached from behind on a flying lap, forcing him to take evasive action.
Leclerc had been set to line up third for his home race, but will now drop to sixth. Norris will start 10th for Sunday's race.
The FIA stewards said Leclerc "had finished his final lap of Q3 and was in the Turn 4 through Turn 10 complex. Norris was on a fast lap and caught Leclerc in the middle of the tunnel and was clearly impeded."
They added: "Both drivers agreed that there was little that Leclerc could have safely done in the tunnel to avoid impeding Norris, given the difficulty in vision due to the light entering and in the tunnel and the change of lines from one side of the tunnel to the other.
"In fact, the Stewards observed that Leclerc reacted in a sensible way to a blue flag displayed by the marshals, but at this point it was too late."
The FIA document added that the stewards had reviewed Leclerc's radio and found Ferrari had failed to give him any warning about Norris' approach until Norris was already directly behind him.
They said: "Further, the discussion during the preceding portion of the track was entirely about competing drivers, not the traffic behind, which is a critical task at this track.
"The Stewards believe that there is much that Leclerc could have done prior to the tunnel to avoid the impeding had he received warning from the team at an appropriate time, especially considering that Norris' approach was clear on the marshalling system."
After the incident, Norris said: "With Charles's incident, it's always tough around here. I guess it did ruin my lap. But the stewards obviously deemed it necessary, I guess.
"I think he did what he could. But when you listen to the team radio, they're just telling him about Verstappen's lap and giving him live updates, rather than concentrating on who's behind him, which I don't really understand. But it is what it is."
McLaren team principal Andrea Stella added: "These kinds of incidents can happen. Unfortunately, Charles was on the racing line, possibly might have not been informed.
"We just reported what we thought the incident was about. It changes our afternoon possibly by a couple of positions.
"But we know at the same time that it's tough for everyone, and I suppose he was just not informed by the team. Because otherwise it was a little too obvious. to be on the racing line under the tunnel, it's very dangerous."
Speaking to Sky earlier in the afternoon after qualifying, Norris said that he had been impeded and said Leclerc had broken the "one rule" about slowing in the tunnel.
He said: "He blocked me, simple as that. A penalty for him. I don't know, disqualified or something?
"There's one rule, which is don't slow down in the middle of the tunnel, and that's what he did, so not up to me."
