The pair, who met in around 2009, announced their engagement in 2013 and had their daughter together the following year. They never married.

"To our daughter Kaya, I will always speak of her mother with respect and make sure she remembers the person she was," he continued.

"I extend my deepest condolences to Hayden's parents, her friends, and her fans, who are also grieving today. Young and talented people like Hayden should not leave this world so soon.

"I ask everyone to please respect our family's feelings and privacy during this incredibly difficult time. For now, this is all I wish to say.

"Thank you to everyone who is supporting my family and expressing their sincere condolences.

"Rest easy, Hayden. May you find eternal peace."

Panettiere met Klitschko, who was 13 years older than her, at a party in Los Angeles when she was 19.

After their daughter was born, the actress was hit hard by postpartum depression and was drinking heavily.

Klitschko helped get her treatment but when their daughter was three, he asked her to sign over custody.

Panettiere reluctantly agreed and wrote in her autobiography, which was published earlier this year: "Not being under the same roof with her every day has been the most gut-wrenching experience of my life, and it's hard to describe the layers of emotion, including sadness, resentment, and anger I felt because of it...

"I grieve that I'm not the mother I thought I'd be, and definitely not the mother I want to be. Trust me, no one should ever have to raise a child on FaceTime."

She remained involved in her daughter's life and added that "despite it all, the bond I have with Kaya is incredibly strong".

Since the news of Panettiere's death broke on Monday, tributes have poured in from her fellow actors, friends and followers.

Connie Britton, her co-star in the musical drama TV show Nashville, posted pictures of them together on-stage on Instagram., external

"This was Hayden. Bursting with brilliance, shining so bright," she wrote in the caption.

"I have no words for this sad song. Just heartbreak," she added. "I love you girl. Sleep easy now."

Masi Oka, who earlier appeared alongside her in supernatural drama series Heroes, said he was "absolutely heartbroken" over the news.

"I had the privilege of working closely with her, and I'll always remember her for her beautiful spirit, immense talent, and the joyful energy she brought to everything she did and the people around her," he wrote on X, external.

"She was way wiser beyond her years and now gone too soon... My heart goes out to her family and everyone who loved her. Rest in peace, Hayden."

Before landing her big roles in Heroes and Nashville, Panettiere had been a child star featuring in films like Remember the Titans and Raising Helen.

Beth Chamberlin, who played her on-screen mother in the US soap opera Guiding Light for three years, described her as "the most brilliant child actor I've ever seen".

"The things she could do, the way she could remember 20+ pages of dialogue, day after day," she posted online, external.

"Even on days where the director would come out between dress [rehearsal] and tape and make huge cuts to the scenes, Hayden, never once forgot a line or missed her blocking. The adults would, but not Hayden.

"I felt blessed to work with her. And always perfect in her playing of the scene."