In a letter published in the Sunday Times ( 15^th December), academics said: “As social statisticians, quantitative social scientists and epidemiologists, we are concerned about the proposed online guidance to accompany the sex question in Britain’s 2021 census, which advises respondents that they may respond in terms of their self-identified gender.”

The census in England and Wales will retain a binary male/female sex question and add a separate voluntary question on gender identity for respondents aged over 16.

Under the proposed guidance for the sex question, respondents can give a different answer to what sex is on their birth certificate.

The letter, sent to census authorities, has 74 signatories from institutions including University College London (UCL), Bristol University and the British Academy.

According to the academics, the guidance will “effectively transform the sex question into one about gender identity”. They expressed concern that it would “undermine data reliability on a key demographic variable” and affect their ability to measure sex-based discrimination and inequality.

The decision to include a voluntary question on gender identity was welcomed by the signatories, but they added: “Sex and gender identity are distinct and should not be conflated.”

An Office for National Statistics spokesperson said: “We are continuing to ask the binary choice male/female sex question. This approach is unchanged since 1801.

“For those who wish to tell us about different gender identities, we propose adding an additional voluntary question on this subject for people aged 16 and over. The entire suite of questions work together to provide the data required for provision of vital public services.

“Most people will not need guidance for the sex question but for those who do, the guidance we are proposing is very similar to 2011. We sought, and continue to seek, a wide range of views in putting this together.”

In September, a group of social science researchers asked the Scottish parliament to ensure that the census asks people to report their legally recognised, rather than self-identified, sex.