The couple were together on and off for nine years and had a daughter, Kaya, who is now 11. She moved to Europe with her father, who was granted custody when her parents separated. The couple were engaged, but never married.
"I deeply mourn Hayden's tragic death. She left the world too soon. Although we were no longer together, she was an important part of my life and the mother of my daughter Kaya."
"Nothing can take away the time we had together and the place she had in our lives," writes Ukrainian Klitschko, who is the brother of Kyiv Mayor Vitali Klitschko.
Hayden Panettiere died on Sunday, aged 36.
