Much of Britain on Sunday conducts a once-in-a-decade census, for the first time completed primarily online and which will ask about gender identity to help develop policy, services and "further equality".
The census will be held in England, Wales and Northern Ireland to gather detailed information on the country's population, their characteristics, education, religion, ethnicity, working life and health.
It was postponed in Scotland until next year due to the pandemic, so UK-wide results will be collated later.
The census - the 22nd in British history and last conducted in 2011 - is carried out to help inform decision-makers in national and local government, community groups, charities and businesses.
The compulsory survey is the most important single source of information about the size and characteristics of the country's population.
Traditionally conducted on the doorstep by an army of questioners, this year it will be completed primarily online, in a change spurred by the coronavirus crisis.
Around 90 percent of households have been sent a code to login with online, while some paper copies are also available for those who need them.
People refusing to complete the survey face a fine.
The census has previously asked people for their sex, among other things.
However, respondents aged over 16 can also now provide a voluntary response to an additional question about whether "the gender you identify with (is) the same as your sex registered at birth".
'Unlawful'
It follows new legislation approving the move, "to meet the user need for better quality information for equality monitoring and to plan and provide services," according to the Office for National Statistics (ONS).
The agency, which conducts the survey in England and Wales, noted there are currently no official figures for those who identify their gender as different from the sex registered at birth.
It agreed to issue guidance in answering the gender questions following a High Court ruling over legal action brought by campaign group Fair Play For Women ahead of this year's survey.
The group had argued posing the secondary question asking what gender a respondent identified with was "unlawful" and "sex self-identification through the back door".
The ONS agreed to make clear in its guidance that the first question about sex can only be answered by reference to a birth certificate or gender recognition certificate.
Several other countries including Nepal and Bangladesh have added a "third gender" option to censuses for people who are transgender.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
