The number of transgender people living in the UK may have been overestimated by the 2021 census, the Office for National Statistics (ONS) has confirmed.

Following the survey in England and Wales, it was reported that around one in 200 people aged over 16 self-identified as a different gender to their sex at birth.

However, a review found that people who do not speak fluent English may have misunderstood the wording of the relevant question and mistakenly answered that they consider themselves to be trans.

The ONS had previously defended its methodology, despite some academics raising concerns over its accuracy.

People who filled out the 2021 census in England and Wales were asked: “Is the gender you identify with the same as your sex registered at birth?”

According to the ONS, 262,000 people – around 0.5% of the population – answered “no”.

But on Wednesday, Mary Gregory – a deputy director at the ONS – said some people may have misunderstood the question.

Writing on the ONS’s website, she said there was “potential bias” in how the question was answered “by those who responded that they had lower levels of English proficiency, some of whom may have mistakenly given an answer suggesting they were trans”.

The number of people who answered the question mistakenly is thought to be “relatively small”, she added, but enough to paint an inaccurate picture in areas “where there are higher levels of non-English speakers”.

Michael Biggs, a professor of sociology at Oxford University, publicly questioned the statistics in April.

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Writing in the British Sociological Society journal, he noted a surprisingly high proportion of people recorded as identifying as trans did not speak English (or Welsh in Wales) as a first language.

While only 10% of people said they did not speak English as their main language on the census, it appeared they made up 29% of the total number of transgender people, Prof Biggs said.

For example, the London Borough of Newham had the highest proportion of people who identified as transgender (1.51%). The area also has one of the highest rates of non-English speakers – 35%, compared to 9% nationally.

The Office for Statistics Regulation has downgraded the data from “accredited official statistics” to “official statistics in development” to reflect the possible flaws.

The regulator said other statistics produced by the 2021 census are still considered to be fully accurate.

The ONS had previously said its estimates on the transgender population were in line with “international comparators”.