EHRC Equality Act (2010) protected characteristic definition - EHRC statement: Protecting people from sex and gender reassignment discrimination (4 April 2022):

In the Equality Act 2010 the protected characteristic of sex protects people from being discriminated against because of being a man or a woman (Equality Act 2010, Section 11) – defined as a male or female of any age (Equality Act 2010, Section 212 (1)). ‘Sex’ is understood as binary – being male or female – with a person’s legal sex being determined by what is recorded on their birth certificate, based on biological sex. A trans person can change their legal sex by obtaining a Gender Recognition Certificate through procedures set out in the Gender Recognition Act 2004. A trans person who does not have a Gender Recognition Certificate retains the sex recorded on their birth certificate for legal purposes.

Under the Equality Act people are protected from sex discrimination on the basis of their legal sex. This means that a trans woman who does not hold a Gender Recognition Certificate is legally male and is treated as a man for the purposes of the sex discrimination provisions, and a trans woman with a Gender Recognition Certificate is treated as a woman. The sex discrimination exceptions in the Equality Act therefore apply differently to trans people with and without Gender Recognition Certificates. 

EHRC: Gender Pay Gap: The gender pay gap is the difference in average pay between the men and women.

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