Ashton Kutcher has stepped down from his role at Thorn, an anti-child sex abuse organization he co-founded, after drawing criticism last week over letters of support he and his wife Mila Kunis wrote on behalf of their former "That 70s Show" co-star Danny Masterson ahead of a sentencing hearing.

Kunis is also stepping down as an observer on the Thorn board, according to a report by Time.

"After my wife and I spent several days of listening, personal reflection, learning, and conversations with survivors and the employees and leadership at Thorn, I have determined the responsible thing for me to do is resign as Chairman of the Board, effectively immediately. I cannot allow my error in judgment to distract from our efforts and the children we serve," Kutcher wrote in his resignation letter to the board, shared by the organization on its website Friday.

Masterson was convicted in May on two counts of rape. He was sentenced last week to 30 years to life in prison.

Kutcher co-founded Thorn in 2009 with his ex-wife Demi Moore. The group works to end the sexual exploitation of children and human sex-trafficking.

"Victims of sexual abuse have been historically silenced and the character statement I submitted is yet another painful instance of questioning victims who are brave enough to share their experiences," Kutcher noted in his letter to the board. "This is precisely what we have all worked to reverse over the last decade."

Kunis and Kutcher were among nearly 50 people to offer character references and advocate for leniency for Masterson.

Masterson has maintained his innocence and plans to appeal his case.

After the letters to the court became public, the couple said in a video shared on Instagram they were "aware of the pain" they had caused.

Kutcher said their intention with the letters was "not to undermine the testimony of the victims or re-traumatize them in any way. We would never want to do that. And we're sorry if that has taken place."

"We support victims, we have done this historically through our work and will continue to do so in the future," Kunis said in their video.

In his resignation letter, Kutcher said he wanted the focus of Thorn to remain on the organization's mission to help victims and survivors.

"I want to offer my heartfelt apology to all victims of sexual violence and everyone at Thorn who I hurt by what I did," Kutcher wrote. "And to the broader advocacy community, I am deeply sorry. I remain proud of what we have accomplished in the past decade and will continue to support Thorn's work. Thank you for your tireless advocacy and dedication to this cause."

Thorn acknowledged Kutcher's contributions to the organization over the past 15 years in a statement on its website, adding, "While the last few days have challenged us all in new ways, we remain steadfast in our mission, mindful of the trust our community places in us and confident in our collective dedication to the children we serve."