The Court of Appeals is today hearing the case of a campaigner who wants the government to introduce an 'X' marker to passports for people who do not identify as male or female.
Christie Elan-Cane's campaign to change the gender markers in passports failed at the High Court last year. They had argued at the time that the current policy of Her Majesty's Passport Office breached human rights laws.
At present, applicants for a UK passport can only choose from male or female.
Elan-Cane's lawyer Kate Gallafent argued that the policy breached their right to private life under Article 8 of the European Convention of Human Rights and the right to non-discrimination on grounds of sex or gender under Article 14.
But the High Court refused to rule the policy unlawful, with lawyers for the home secretary arguing that it maintained an "administratively coherent system for the recognition of gender".
Speaking ahead of Tuesday's hearing, Elan-Cane told the BBC that the options were "inherently discriminatory" and "unacceptable".
"Legitimate identity is a fundamental human right but non-gendered people are treated as though we have no rights," they said.
"It is unacceptable that someone who defines as neither male nor female is forced to declare an inappropriate gender in order to obtain a passport."
A number of countries already allow a gender neutral passport, including Canada, Germany, Australia and New Zealand.
Last year, the Netherlands issued its first gender neutral passport to a person who identifies as intersex.