Fanning, who is keeping up a gruelling schedule of movie-viewing and soirees at the glitzy French Riviera event, said on Instagram Monday: “Oops, had a fainting spell tonight in my 1950’s Prada prom dress but it’s all good.”

She posted a selfie giving the thumbs up sign, adding the hashtags #dresstootight and #timeofthemonth.

British actor Colin Firth, who was attending the same dinner, rushed to Fanning’s aid until the “Beguiled” star was taken away for medical assistance, Variety said.

Fanning is one of nine film industry luminaries judging this year’s selection at the world’s top film festival.

They will announce who has won the coveted Palme d’Or for best film on Saturday, choosing one of 21 contenders including Quentin Tarantino’s latest, “Upon Upon a Time In Hollywood”, which premiered on Tuesday.