The "sex" question in the next census could be made voluntary after claims it discriminates against transgender people.

The "tentative" recommendation has been made in an Office for National Statistics (ONS) report on gender identity.

The move would make the UK one of the first countries in the world not to require its citizens to tell officials what sex they are, according to the Sunday Times.

The proposed change for the 2021 census could leave Britain without an accurate figure for the number of men and women living in the country.

Research carried out by the ONS found the "sex" question included on the 2011 census, which requires respondents to choose whether they are male or female, "was considered to be irrelevant, unacceptable and intrusive, particularly to trans participants, due to asking about sex rather than gender".

The report explored other options, including a "hybrid" question with the addition of an "other" category.

But asking about "sex" was again thought to be "irrelevant and intrusive", while the "other" option was "thought to homogenise trans people and differentiate them from the rest of society," the report said.

A third "two-step" option, with separate sex and gender identity questions, was also ruled out.