Olivier Ciappa, one of the stamp's designers, says Inna Shevchenko was the primary model for Marianne, a fictional heroine, Radio France International reported Tuesday.
Shevchenko, 23, who has been granted political asylum in France, is the leader of the French branch of the self-described radical feminist group Femen, which is best known for topless protests.
Marianne, who is always pictured bare-breasted, is most famously known as the woman waving a French flag in the painting "Liberty Leading the People" by Eugene Delacroix.
Ciappa said he chose Shevchenko because he believed Marianne probably would have been a member of Femen "because she fought for the Republic's values -- liberty, equality and fraternity."
The leader of France's Christian Democratic Party on Monday called for a boycott of the stamp. Critics of Shevchenko consider her a narcissistic, divisive figure who does not understand women's issues in France.
Shevchenko said she was unaware she had inspired the image on the stamp, which was unveiled Sunday by President Francois Hollande.