Ashton Kutcher and his wife Mila Kunis have resigned from the board of the anti-child sexual abuse charity he founded amid swirling criticism of their defense of fellow actor and convicted rapist Danny Masterson.

Kutcher told the board of Thorn he was standing down immediately as its chair in a letter written on Thursday and posted to the organization’s website. Kunis’s role as an observer on the board ended at the same time.

Actors Kutcher, 45, and Kunis, 40, starred with Masterson in the popular teen comedy That 70s Show in the late 1990s and early 2000s, and they wrote glowing character references for him during his trial in May for raping two women in 2003.

Masterson was convicted and sentenced last week to 30 years in prison, despite Kutcher’s depiction of his friend as a man “with decency, equality, and generosity”, and Kunis’s praise for him as “an outstanding role model and friend” and “exceptional older brother figure”.

The pair made a public apology in an Instagram video over the weekend, saying they were sorry for the “pain” their letters had caused and expressed regret for undermining the testimony of the victims.

But critics questioned the appropriateness of the couple’s ongoing role with Thorn, which Kutcher founded with his former wife Demi Moore in 2012, and which has become a powerful voice in the campaign to end the sexual exploitation of children globally.

In his letter, Kutcher apologized again and said he “cannot allow my error in judgment to distract from our efforts and the children we serve”.

“The mission must always be the priority and I want to offer my heartfelt apology to all victims of sexual violence and everyone at Thorn who I hurt by what I did,” he 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.”

Responses from several of Kutcher’s now former colleagues appeared below the letter.

“His unwavering dedication and commitment to Thorn throughout its journey have enabled the organization to become the leader that it is in the child safety ecosystem,” board member Suzanne Bell wrote. “It has been my privilege to join him on this mission.”

The charity said Kutcher had a “significant impact” on its work, which it said included helping the tech industry remove more than 2m potential child sexual abuse files from the open web.

“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,” it said.

• Information and support for anyone affected by rape or sexual abuse is available from the following organisations. In the US, Rainn offers support on 800-656-4673. In the UK, Rape Crisis offers support on 0808 500 2222. In Australia, support is available at 1800Respect (1800 737 732). Other international helplines can be found at ibiblio.org/rcip/internl.html