UK passports should allow those who do not identify as male or female to define themselves as "X", a leading LGBT rights group has said.
Stonewall said the third option should also be on official government forms and would help trans people who identify as non-binary avoid longstanding difficulties.
"Not having legal recognition means non-binary people must constantly live as someone they are not," the charity said in a five-year plan, published on Wednesday, (5 April) to create equality for transgender people in Britain.
The project, titled A Vision for Change, was created by the Stonewall Trans Advisory Group (Stag) and comes days after HSBC said its customers would be able to choose non-gender specific titles such as Mx, M and Misc for their bank accounts.
Tara Stone, a member of Stag, told BBC Radio 4's Today programme: "For trans people who identify as non-binary – that is, neither male or female, or some combination thereof – it would give them an option which wasn't prescriptive, whereas the only options at the moment are prescriptive for them. It basically allows them to express their identity."
Asked if the "X" option would cause difficulties for those travelling to countries with poor records on LGBT rights, Stone said: "It's important that we have that option but that good advice is given by via the Home Office for people travelling.
"We do that to a certain extent now. It's just a case of having the information there to make people aware so they can make good, informed choices."
There are an estimated 650,000 people who identify as trans in the UK.
The UK government says it is reviewing the way gender is marked in official documents, including passports.
Other countries like Australia already allow their citizens to choose "X" if their gender does not fit into the male or female categories.
Stonewall said more should also be done to make it easier for trans people who identify as a gender different from birth to change legal documents like passports.
It said many are currently "afraid to travel abroad" because they fear intrusive questioning or difficulties at passport control, adding those travelling with children were especially affected, particularly if they were born or adopted when they were legally a different gender.
Stonewall's plan also includes proposals for lobbying for a reformed Gender Recognition Act and Equality Act to incorporate trans-inclusive policies.