Ashton Kutcher stepping away from an anti-child sex abuse organization he co-founded.

Amid public backlash over letters he and wife Mila Kunis wrote in support of their That '70s Show costar Danny Masterson ahead of his rape case sentencing, the Punk'd alum has resigned from his post as chairman of Thorn's board on Sept. 15.

"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," Kutcher wrote in his letter of resignation, which was published in full by Time. "I cannot allow my error in judgment to distract from our efforts and the children we serve."  

He further acknowledged that his letter in support of his longtime friend—who was ultimately sentenced to 30 years to life in prison—undermined the organization's efforts.

"As you know, I have worked for 15 years to fight for people who are sexually exploited," the No Strings Attached star continued. "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. This is precisely what we have all worked to reverse over the last decade."

Offering a "heartfelt apology to all victims of sexual violence and everyone at Thorn who I hurt by what I did," as well as the "broader advocacy community," Kutcher added, "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."

Kutcher and then-wife Demi Moore launched Thorn in 2012 after learning about "the issue of child sex trafficking from a documentary highlighting what was happening to children in Cambodia," according to the organization's website. As part of his role as co-founder, he testified at a 2017 hearing with the Senate Foreign Relations Committee to examine the United States' progress in ending modern slavery and human trafficking.

The 45-year-old's resignation from Thorn comes a week after letters he and Kunis, 40, penned to a judge presiding over Masterson's sentencing were made public. In documents obtained by E! News, Kutcher described Masterson—who was convicted on two counts of rape in May but has denied any wrongdoing—as an "excellent" role model who had been "nothing but a positive influence on me."

"He has always treated people with decency, equality, and generosity," he wrote, before urging the judge to take Masterson's 9-year-old daughter Fianna Francis with wife Bijou Phillips into consideration in his sentencing. "I do not believe he is an ongoing harm to society and having his daughter raised without a present father would a tertiary injustice in and of itself." 

In the face of criticism over their support for Masterson, Kutcher and Kunis released a video message on Sept. 9 to apologize for "the pain that has been caused by the character letters that we wrote on behalf of Danny Masterson."

"They were intended for the judge to read and not to undermine the testimony of the victims or retraumatize them in any way," Kutcher said of the letters. "We would never want to do that, and we're sorry if that has taken place."