Italy's agency for flight security, ANSV, said its investigation had found the airline that owned the plane was not fully licensed to operate and the pilot's licence had expired on November 30 – more than a month before the flight took place on January 5.
The plane carrying Mr Missoni, 58, his wife, Maurizia Castiglioni, another couple, a Venezuelan crew member, and the pilot – who was in his early 70s – disappeared after taking off from the resort of Los Roques, an archipelago off the coast.
Italy has sent rescue personnel to help with the search for the lost plane.
Mr Missoni is the oldest child of the founders of the fashion house famous for its exuberantly coloured knits, featuring bold stripes and zigzags.
He is co-owner with siblings Luca and Angela, who handle the technical and design sides of the firm.