A High Court judge has ordered that part of the guidance from the UK Census accompanying the question on an individual’s sex should be removed.

Campaign group Fair Play For Women launched legal action over the guidance, introduced into the 2021 Census, arguing that it is “unlawful” and describing it as “sex self-identification through the back door”.

The question “What is your sex?” on the survey, which is conducted by the Office for National Statistics (ONS) every 10 years, is accompanied by guidance to help people with their response.

The guidance reads: “If you are considering how to answer, use the sex recorded on one of your legal documents such as a birth certificate, gender recognition certificate, or passport.

“If you are aged 16 years or over, there is a later voluntary question on gender identity.

“This asks if the gender you identify with is different from your sex registered at birth.

“If it is different, you can then record your gender identity.”

Following a hearing on Tuesday, Mr Justice Swift ruled that the online guidance, which is not in the paper versions of the census, should be altered to make it clear that the question can only be answered by reference to a birth certificate or gender recognition certificate.

The judge also gave the campaign group permission for a full judicial review of its claim, which will be heard urgently because the census is due to go live on March 21.

Fair Play For Women, which has crowdfunded £100,000 to bring a legal challenge against the guidance, had argued that withdrawing the guidance now would be the “safer option”.

At Tuesday’s hearing, Jason Coppel QC, representing the group, said the guidance “conflates and confuses” sex with gender identity.

He told the court that a person’s sex on a passport, or other legal document such as a driving licence, can be altered without a formal legal process.

The barrister argued this could potentially have the effect of “distorting” the data the ONS gathers through the census.

Lawyers for the ONS argued that, in the context of the “What is your sex?” question, sex is an “umbrella term” which could refer to a person’s sex as recorded in any document provided by the state.

But, granting Fair Play For Women’s application, Mr Justice Swift said he was satisfied the campaign group has a “strongly arguable case” on the issue.

The judge said there was a “mismatch” between the way the guidance was phrased and the legislation which sets out how the Census must be conducted.

He also said he was satisfied the campaign group is “more likely than not to succeed” on its case regarding the legal meaning of sex as defined in the legislation.

Dr Nicola Williams, director of Fair Play For Women, said in a statement ahead of the hearing: “Accurate data on sex matters.

“It matters most to women and girls.

“We need it.

“If we don’t have good data on sex we can’t monitor inequalities due to sex, and if we can’t measure it we can’t make good policies to remedy it.

In a statement posted on its website in February, the ONS said: “We are continuing to ask a binary choice male or female sex question on the census.

“This approach is unchanged since 1801.

“There is a new voluntary question on gender identity for people aged 16 years and over later in the questionnaire.

“We have worked closely with organisations and individuals from a wide range of representative groups over five years to ensure the Census questions we ask provide the best information possible.

“As with previous censuses, most people will not need help to answer the sex question, but for those that do, guidance will suggest they use sex as recorded on legal documents such as a birth certificate, gender recognition certificate or passport.

“By referring to ‘legal documents’ the guidance makes clear we are referring to government-issued documents.

“This is not self-identification, which was evaluated as part of a range of options but not taken forward.”