Flag of the Vatican City, as described in the 2000 w:en:Fundamental Law of Vatican City State (which took effect in Feb. 2001), Article 20, Attachment A. See here and here.
Own work using: http://files.mojeeuro.meu.zoznam.sk/200000288-390ab3a04d/2_Commemorative_coin_Vatican_city_2010.jpg
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