Actress Mila Kunis, left, and actor Ashton Kutcher, right, sit courtside together at the NBA basketball game between the Phoenix Suns and Los Angeles Lakers on Tuesday, February 12, 2013, in Los Angeles. (AP)

Ashton Kutcher and Mila Kunis have stepped down from their roles on the board of the anti-child sexual abuse charity founded by Kutcher, amid growing criticism of their support for fellow actor Danny Masterson, who was convicted of rape.

Kutcher, who served as the chair of Thorn, announced his immediate resignation in a letter posted on the organization's website, while Kunis, who held an observer role on the board, also concluded her involvement.

The couple, known for their roles in "That '70s Show" alongside Masterson, had provided character references for him during his trial for the rape of two women in 2003. Despite their support, Masterson was sentenced to 30 years in prison.

Kutcher and Kunis publicly apologized in an Instagram video, expressing regret for the distress their letters had caused and acknowledging that they had undermined the victims' testimonies.

Critics raised concerns about the appropriateness of the couple's continued association with Thorn, an organization founded by Kutcher and his former wife, Demi Moore, in 2012. Thorn has played a significant role in the global campaign against child sexual exploitation.

In his resignation letter, Kutcher offered another apology, emphasizing that he could not allow his lapse in judgment to divert attention from the organization's mission and the children they serve. He expressed profound remorse for hurting victims of sexual violence and the Thorn community.

The charity recognized Kutcher's substantial contributions, including his assistance in removing over 2 million potential child sexual abuse files from the open web within the tech industry. Thorn reaffirmed its commitment to its mission despite recent challenges, acknowledging the trust placed in them by the community.