Research and debate on transgender issues is being stifled at British universities, a letter signed by over 30 academics has claimed.

In a letter to the Sunday Times, the professors said the relationship between British universities and the LGBT lobby group Stonewall was "inappropriately close", while staff training run by the charity is "in tension with academic freedom".

Signatories of the letter include Professor Rosa Freedman, of the University of Reading, who says she has received rape and death threats from trans rights activists over her opposition to gender self-identification.

The academics say that university policies are advised to stand against "transphobic" teaching or research, but the term is not clearly defined, leaving academics feeling that they are "unable to question the contested notion of 'gender identity' without fear of sanction".

The letter, signed by academics from Oxford, Cambridge and other top institutions, adds that under the guidance, universities should refrain from allowing speakers onto campus who do not agree with the view that people can be the gender they choose.

"This is a further unacceptable restriction upon free academic debate," they said, adding that "tendentious and anti-scientific claims are presented to academics as objective fact, without the opportunity for scrutiny".

Signatories of the letter said they want the freedom to explore the ramifications of some transgender views, such as opening up female-only spaces to those who identify as women, and how to approach gender-questioning by children.

"It is imperative to interrogate the radical shifts in thinking that all this implies, but we feel inhibited from doing so in the intimidating atmosphere produced by Stonewall's influence," they said.

Stonewall has defended its diversity training programme at universities, saying: "Our work with universities is vital: we know LGBT staff and students experience discrimination daily."

The LGBT+ Network of Networks in Higher Education, a group of academics and researchers, accused the signatories of the Sunday Times letter of spreading a "conspiracy theory" and said in response that they "entirely reject" the claim that "promoting an educational environment free from harassment and bullying via the Stonewall Diversity Champions programme contradicts the principles of academic freedom".

"The letter muddles the distinction between the concept of academic freedom, and the statutory public sector equality duties of higher education institutions," the group said.

"For the former, the right to conduct academic debates, research, and publish activities without fear of retribution by those in power, is the cornerstone of the academy.

"Whilst the latter relates to the institutional duties towards legally protected groups to eliminate discrimination, harassment and victimisation, promote equality of opportunity and foster good relations through the practices of higher education institutions.

"Debating gender pronouns within an academic context is part of academic freedom whereas the refusal to respect the gender pronouns of an individual to be used in class is not."