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Ashton Kutcher has resigned from the charity he set up to tackle child sexual abuse, following outrage over a letter of support to a judge after Danny Masterson's rape conviction.

In his resignation to the charity, Thorn, Kutcher said his support letter was an "error in judgement".

Masterson was sentenced to 30 years to life in prison for the rapes of two women.

A similar letter was sent by Kutcher's wife, actress Mila Kunis.

The letters and subsequent apology from Kutcher and Kunis - both of whom starred in That 70s Show alongside Masterson - were widely criticised by victims and advocacy groups.

In a resignation letter directed to the board of Thorn and posted online, Kutcher said his decision to step down from the board is "rooted in the recognition of recent events".

"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, effective immediately," Kutcher said in the resignation letter.

"I cannot allow my error in judgment to distract from our efforts and the children we serve," he added.

Established by Kutcher and ex-wife Demi Moore in 2012, Thorn focuses on combatting sexual exploitation of children.

Much of the group's focus is on the Internet and the role it plays in the spread of child sexual abuse material and sexual slavery. To date, the group claims to have identified 27,000 child victims and removed more than two million potentially sexual abuse files from the internet.

In his resignation message, Kutcher said his letter of support for Masterson - in which he described him as someone who treated people with "decency" and "generosity" - was "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," he added.