The NSPCC is facing an internal revolt over its decision to cut ties with the trans activist Munroe Bergdorf in the face of social media criticism of her appointment.

Bergdorf, a model and former adviser to the Labour party, announced last week that she was proud to have become the first LGBT+ campaigner for Childline, the child protection charity’s counselling service.

After criticism on social media that Bergdorf was an inappropriate “sexualised” role model for children, and accusations that she was a “porn model” because she once posed for Playboy, the NSPCC announced 48 hours later that it would have “no ongoing relationship” with her.

In an internal memo sent on Monday, the charity’s CEO, Peter Wanless, acknowledged “a great deal of public feeling” and added: “Of course we regret any hurt caused to Munroe Bergdorf.”

But in a letter seen by the Guardian, nearly 150 members of the charity’s staff spoke of their “embarrassment and shame” and condemned the decision “to cut ties with Munroe Bergdorf, a black trans woman, as a Childline campaigner.”

They wrote: “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.”

The employees – said to be working in a “sombre” atmosphere after the decision – said they were alarmed that the statement provided by the NSPCC failed to articulate the reasons for the move and to “express solidarity with Munroe in the face of significant online abuse”.

They went on: “Overall, there is a feeling of embarrassment and shame about how this has been handled and what it means to be an employee of the NSPCC.”

The charity had faced criticism from some on social media for working with Bergdorf, including from an account called Safe Schools Alliance UK, which was formed in May, and from the Times columnist Janice Turner. The Safe Schools Alliance account said Bergdorf had invited children to get in touch with her directly on social media and was “supportive of children dressing as ‘drag queens’”.

Turner asked on Twitter whether the charity’s decision to recruit “a porn model” as a campaigner was “worth the cancelled direct debits”, saying that by porn model she meant “modelling for porn title Playboy”. Bergdorf posed for the magazine in 2018.

Bergdorf’s supporters have highlighted that the NSPCC has previously worked with the glamour models Melinda Messenger and Abbey Clancy.

Bergdorf said in a statement that she had never appeared in porn but that demonising those who had was unacceptable. She said she was “unbelievably sad” that the charity had decided to “bow down to anti-LGBT hate and overt transphobia”.

She said the NSPCC had “decided to sever ties without speaking to me, delete all the content we made together and backpedal without giving any reason why”.

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.”

The letter from NSPCC staff was signed by 148 people by the time it was sent on Monday afternoon. Members of staff told the Guardian that the mood among some employees was “pretty sombre” when they saw the charity’s statement.

“[Some of us were saying:] ‘I feel ashamed, ashamed to be working here’,” said one staff member, speaking to the Guardian anonymously. “None of us have said that before. We are generally very proud of what we do …people are just pretty sickened.”

They said the fact that it came during Pride month was “the worst possible timing”. “We are worried about how we show our support for young people at Pride,” they said. “This reflects on us. The decision of our trustees reflects on us, but it’s an inaccurate impression.”

They said it appeared as if the trustees had bowed to pressure from “a relatively small group of people” who had been in touch to lobby them. “If the trustees make decisions like that without consultation from people who have lived experience or who know this work, then where does that end?”

In an internal memo to staff seen by the Guardian, Wanless said “a great deal of public feeling is being expressed about Childline” and added: “It is vital that we don’t let this distract from our fundamental purpose to be there for all children.”

He said: “I am very grateful to all staff who have been working on this issue over the weekend. Of course we regret any hurt caused to Munroe Bergdorf following her support of our most recent Childline campaign. Those involved will take learnings from this situation going forward.”

Bergdorf told the Guardian: “I never doubted the hard work of the Childline staff on the ground, especially their LGBT+ employees who have been working to make the charity more inclusive. I want to say thank you for their support. I’m hoping that this is a teachable moment in how transphobia poses a real threat to the progression of our community and our individual emotional wellbeing and livelihoods.”

She said she saw nothing wrong with letting trans children message her “as a friend to turn to” and argued that drag was a “cultural phenomenon” that had no age limit. “There is absolutely nothing wrong with kids playing dress-up if that’s what they want to do,” she said.

The NSPCC did not respond to a request for comment.