According to Wikipedia:

It is often suspected that the name Marie-Claire is inspired by the originally French Marie Claire magazine, a women's fashion weekly first published in 1937. One theory says that this song is about the Italian star Sophia Loren, who was abandoned by her father and had a poverty stricken life in Naples. Another theory has the song being inspired by singer and actress Nina van Pallandt. In reality, Peter Sarstedt wrote the song about a girl he fell madly in love with in Vienna in 1965. She died tragically in a hotel fire.[citation needed] The song was written in Copenhagen. (Confirmation of this fact can be found on the CD cover of "The Best of Peter Sarstedt", EMI, nr. 8297622, Australian CD).