Five judges at Britain's Supreme Court on Wednesday rejected a legal challenge against a government policy not to allow gender-neutral passports.

Christie Elan-Cane, who does not identify as either male or female, argued that Britain's passport application process was "inherently discriminatory".

Individuals only have a choice of "male" or "female" on the application, with no option to put "X" for "unspecified" -- an alternative introduced in several other countries.

But the Supreme Court unanimously dismissed the appeal, ruling that a person's identity could be confirmed using the form and checking it against other official documents.

That included birth, adoption or gender recognition certificates, said judge Robert Reed in the ruling.

"It is therefore gender recognised for legal purposes and recorded in those documents which is relevant," he added.

The case was brought on the grounds that the government breached legal rights to a private life and not to face gender or sex discrimination.

The judges, though, said the lack of a gender-neutral option "does not unjustifiably breach articles 8 and 14 of the European Convention on Human Rights".

Elan-Cane, who has campaigned on the issue for 25 years, had earlier lost cases in lower courts.

The activist, who uses the pronouns "per/per/perself", said the government and courts were "on the wrong side of history".

"This is not the end," the campaigner wrote on Twitter, promising to take the case to the European Court of Human Rights in Strasbourg.

Countries including Canada, Australia, Denmark, Germany, Malta, Nepal and Pakistan all now issue passports with options other than male and female.