You asked

Please can you tell me the reasons why you are not using consistent language across the two census questions on sex and gender?

The gender question (which I assume is actually one about gender identity?) asks whether your 'gender' is the same as 'your sex registered at birth'.

So why does the guidance for the sex question not say that this is a question about 'your sex registered at birth'?

We said

Thank you for your email of 18 October 2020 regarding the questions on sex and gender identity in Census 2021.

The sex question is "What is your sex?", with the response options female and male. The question has been asked since 1801 and remains unchanged. It is important that the definitions of key questions are retained in order to maintain comparability over time. Sex is a core demographic variable, of fundamental importance for a range of census users providing vital information for national and local decision making. It is also one of 9 protected characteristics in the 2010 Equality Act and the data are used extensively to monitor and measure inequalities. In our December 2018 topic research update, we recommended that to maintain or improve data quality we would: retain the wording of the sex question and response options used in 2011 and include a note on the sex question stating that a gender identity question would follow later. Regarding the online guidance for the sex question, our experience in our 2019 Census Rehearsal showed that, similarly to the 2011 Census, the majority of people answer this question without accessing the guidance.

The new, voluntary question on gender identity is designed specifically to provide information on the transgender population, i.e. those people whose gender identity is different from their sex registered at birth. Therefore this definition is used in the question, which asks "Is the gender you identify with the same as your sex registered at birth?", with the response options Yes and No (in which case people are asked to write in their gender identity).

You can see the final questions in the Census 2021 paper questionnaires (in the online questionnaire, the guidance note alongside the sex question appears only for people aged 16 or over and omits the age reference used on the paper questionnaire version.) The guidance is still under consideration and yet to be finalised.