The stewards ruled that Webber had broken article 30.9(b) of the sporting regulations because he "entered the track without the marshal's permission between the commencement of the formation lap and the time when the last car enters parc ferme."
Because that was Webber's third reprimand of the year, he will receive an automatic 10-place grid penalty for the next event.
Webber's previous reprimands came for a clash with Nico Rosberg in Bahrain and failing to sufficiently heed yellow flags in Canada.
Alonso was also given a reprimand.
The stewards said he "drove the car in a manner which could be deemed potentially dangerous to other drivers or any other person.
"In this case, two cars had to take avoiding action as a result."
A video replay provided by the FIA showed that Alonso stopped in the middle of Turn 7 after being hailed by Webber.
As Webber ran around to the left of the Ferrari, one of the Mercedes came close to him, while the second had to go around the outside and across the kerbs.