Guidance on the sex question in the UK census must be changed before the official day to complete it on 21 March, a high court judge has ruled. The Office for National Statistics (ONS) had issued new advice on how to answer the question “What is your sex?” on the survey this year, which read: “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.” It added: “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.” However, on Tuesday Mr Justice Swift ordered that the guidance should be rewritten to remove the words “such as” and “or passport”, to make clear that respondents should only use the sex recorded on their birth or gender recognition certificate. A little more than an hour after the judge’s ruling the text had been changed. The campaign group Fair Play For Women, which crowdfunded £100,000 to bring the legal challenge, had argued that the ONS wording allowed “self-identification through the back door”. Jason Coppel QC, representing the group, told the court the guidance “conflates and confuses” sex with gender identity, pointing out 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. This risked “distorting” the data the ONS gathered through the census, he added. Giving permission for the case to proceed to a full judicial review, the judge said there was a “mismatch” between the way the guidance was phrased and legislation that sets out how the census must be conducted. He added that he was satisfied the campaign group had a “strongly arguable case” and was “more likely than not to succeed” regarding the legal meaning of sex as defined in the legislation. Lawyers for Fair Play For Women had argued it would be “safer” to withdraw the guidance, given the shortage of time until census day. Paper copies of the census do not contain the new guidance. The census is taking place in England, Wales and Northern Ireland on 21 March, while Scotland’s census has been postponed for a year because of coronavirus. Respondents are being asked to fill in the online form on that day or as soon as possible afterwards, but the website is already live. The judge expressed surprise that according to the ONS a fifth of households (about 3 million) had already completed the survey when it was “clear” that census day was Sunday 21 March. Dr Nicola Williams, the director of Fair Play For Women, said in a statement before 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 it was continuing to ask a binary choice sex question on the census with the only possible answers being male or female. “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,” said the statement. It continued: “As with previous censuses, most people will not need help to answer the sex question … 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.”