"Last summer I talked a lot about this with other academics and lawyers but hadn't said anything publicly," Prof Freedman said. "A colleague said to me, 'You have a responsibility, you are a human rights lawyer and you understand vulnerable groups, people can learn from you.'" Since then, she has faced death threats, intimidation and harassment - which included having urine poured on her office door - and repeated attempts to get her sacked from her post at Reading University.

It was a similar story for Professor Kathleen Stock, an expert in analytic philosophy at Sussex University, who felt compelled to intervene in the debate last summer. "I was immensely frustrated by what I saw was the shameful silence of philosophers," she said. "The position that 'you are who you say you are' simply doesn't fit with most of western philosophy. There was a public consultation going on and it was clear that people were frightened to speak out. I thought we need to break cover and get this conversation started."

Prof Stock has faced a no-platforming campaign and plenty of online abuse for her views. She has started compiling a dossier of cases where fellow academics have had to self-censor for fear of the backlash they will face from the transgender lobby, or even from their own colleagues.

Universities are looking for customers

One area of particular unease for academics who speak out for sex-based rights is the way in which their university's complaints process has been used against them. Even where they are eventually exonerated, they described how the investigation process itself can be draining and demoralising.

Prof Michael Biggs, a sociologist at Oxford University who is part of a network of academics interested in sex and gender, said that "almost all" of them have faced disciplinary processes as a result of student allegations. "For the most part they are cleared but the process is the punishment. Of course it is very stressful, you have to spend a lot of time preparing a defence," he said.

"What's Orwellian about it is that you have to keep quiet. Once you are the subject of a complaint you are not allowed to talk about it. The point is that there is a lot of harassment that is not seen and a lot of that will make academics scared, they will say 'I do not want to get involved, I will stay silent'. Because I am tenured and Oxford is a much more protected university, I feel I have to take the lead on this. Others are much more vulnerable and it may well be that they cannot get another job if they are accused on transphobia."