The NSPCC has offered the trans activist Munroe Bergdorf a “full, frank and unreserved apology” for abruptly cutting ties with her in the face of social media criticism of her appointment.

The child protection charity announced last week it would have “no ongoing relationship” with Bergdorf, 48 hours after she told her Twitter followers how proud she was to have become the first LGBT+ campaigner for its counselling service Childline.

Bergdorf, a model, said she had not been contacted by the charity before it issued its statement abruptly ending their relationship with her and, in a letter seen by the Guardian, nearly 150 NSPCC employees spoke of their “embarrassment and shame” at their employer’s decision.

In a statement on Wednesday, the NSPCC’s CEO, Peter Wanless, apologised for any hurt the charity had caused. “I want to acknowledge first and foremost that we shouldn’t have cut ties with Munroe Bergdorf in the way we did,” he said.

“We should have been more thoughtful and caring about how we managed our relationship with her at the outset as we were the ones who had reached out to her and brought her into the Childline family in the first place.”

Bergdorf responded to the statement on Twitter. “I appreciate the formal apology, NSPCC,” she said. “It’s definitely not perfect and I can’t tell you the emotional distress that this has brought to my door. But I just want to move on from this whole ordeal.”

Wanless said the decision to cut ties with Bergdorf – which was made by the charity’s board of trustees – was nothing to do with the fact that she was transgender.

He said the decision was made because of “the lack of process that our organisation used when deciding to work with Munroe” and previous comments that provoked concerns around safeguarding and equality.

He explained: “The board decided an ongoing relationship with Munroe was inappropriate because of her statements on the public record, which we felt would mean that she was in breach of our own risk assessments and undermine what we are here to do.”

Bergdorf had been been criticised for inviting children to get in touch with her directly on social media. The NSPCC had also faced criticism on social media that she was an inappropriate “sexualised” role model for children, and accusations that she was a “porn model”.

Bergdorf posed for Playboy in 2018, but said in a statement that she had never appeared in porn. She added that demonising those who had was unacceptable. Her supporters have highlighted that the NSPCC has previously worked with the models Melinda Messenger and Abbey Clancy.

Bergdorf also told the Guardian on Tuesday that she saw nothing wrong with letting trans children message her “as a friend to turn to”.

In his apology, Wanless said the charity had failed to speak to Bergdorf before making the announcement it were cutting ties on Friday.

“I have spoken with Munroe today about the situation and have offered a full, frank and unreserved apology,” he said. “I agreed with her that we condemn all transphobic bullying, harassment and abuse.”

He added that the board of trustees had commissioned an immediate review of the process by which it establishes relationships with supporters.

Bergdorf has previously been fired by L’Oréal after she said in the wake of racist violence in Charlottesville that white people should acknowledge that their “existence, privilege and success as a race is built on the backs, blood and death of people of colour”.

She stood down from the Labour party’s LGBT advisory board last year after newspaper reports revealed comments from her in her early 20s describing suffragettes as “white supremacists”.

The NSPCC’s statement last week said Bergdorf had “supported the most recent phase of Childline’s campaign, which aims to support children with LGBTQ+ concerns”, though she had never been a “Childline ambassador”.

It said: “She will have no ongoing relationship with Childline or the NSPCC. The NSPCC does not support, endorse or authorise any personal statements made by any celebrities who contribute to campaigns.”

A letter, which was signed by 148 NSPCC staff by the time it was sent on Monday afternoon, read: “We are deeply disappointed about the treatment of Munroe by the organisation.

“In particular, we are concerned at the NSPCC’s decision to replicate the experience that many trans children and adults experience in being subjected to abuse and ridicule and subsequently abandoned.”