A leading prison reformer has said prisoners who have committed violent offences against women should not be able to transfer to women’s prisons if they have not legally changed their gender, after it emerged that a transgender inmate had sexually assaulted fellow prisoners after transferring to a women’s prison.

Karen White was on remand for multiple rapes and other sexual offences against women when she transferred to New Hall prison, near Wakefield.

Last week it emerged that after transferring to the female prison, she was accused of four sexual assaults against other inmates between September and November last year, before being moved to a men’s prison.

White admitted to the sexual assault of two inmates when she appeared at Leeds crown court on Thursday to admit to a rape she had committed before she was sent to prison.

Frances Crook, the chief executive of the Howard League for Penal Reform, said vulnerable women were being put at risk by a small number of violent men whose primary interest was harming women.

Karen White. Photograph: Rex/Shutterstock

“It is a very toxic debate, but I think prisons have probably been influenced by some of the extreme conversations and have been bullied into making some decisions that have harmed women and put staff in an extremely difficult position,” she said.

The government is currently carrying out a consultation about reforming the Gender Recognition Act to make the process of changing gender in England and Wales – which can take up to five years – “less intrusive and bureaucratic for transgender people”.

It has stated that “we are not necessarily proposing self-declaration of gender”, but some groups opposed to the changes fear a process of self-identification could give dangerous men posing as trans women access to vulnerable women, such as those in prisons.

Trans rights groups say prisons are already required to scrutinise each request and analyse risks, and argue that transgender prisoners are among the most vulnerable inmates and are let down by the current system.

The Ministry of Justice has apologised in White’s case and said previous offending history was not taken into account. Requests for transfer from prisoners whose legal gender does not accord with their self-identified gender are normally assessed by a transgender case board, “which should consider all previous offending history”, but this was not carried out in this case.

Crook said that during a recent prison visit a governor told her of five prisoners who were identifying as women and had asked for transfer. All had a history of sexual violence against women and had had their requests turned down. “I would say decision-making is patchy,” said Crook. “While permission is being refused in some instances, clearly mistakes are still being made in others.”

She added: “In my view, any man who has committed a serious sexual or violent offence against women, who then wants to transfer but has not gone through the whole process, still has a penis and still has male hormones, should not be put into a women’s prison. There may be a case for having separate provision; that is a debate to be had.”

Pilgrim Tucker, who has led legal action over proposed changes to the Labour party’s policy on the formal inclusion of self-identifying trans women on all-women shortlists, said women campaigning about self-identification had long warned of the risk it posed. “Almost half of trans women prisoners are sex offenders,” she said. “We urgently need to start prioritising the safeguarding of women and girls over the feelings of male-bodied people.”

A government survey has counted 125 transgender prisoners in England and Wales, which is likely to be an underestimate. According to MoJ figures released in response to a freedom of information request by the BBC, 60 of them have been convicted of one or more sexual offences.

A Prison Service spokesperson said: “We apologise sincerely for the mistakes which were made in this case. While we work to manage all prisoners, including those who are transgender, sensitively and in line with the law, we are clear that the safety of all prisoners must be our absolute priority.”