Major airlines in the US are expanding on the traditional 'male' and 'female' gender options in their booking forms to make them more inclusive of non-binary people. 

American Airlines, Delta, JetBlue and United Airlines are among the airlines introducing gender-neutral designations. 

The changes are due to come into effect on June 1 after being agreed by the Airlines for America trade association, The Daily Beast reports. 

A spokesperson for United Airlines said that its booking forms were being changed to allow people to select 'M' for 'male', 'F' for 'female', 'U' for 'undisclosed' or 'X' for 'unspecified'.

The airline told The Daily Beast the new options would allow customers to correspond their flight booking identity with the gender designation on their passports or ID cards. 

The spokesperson added that customers who do not identify with the traditional male or female titles like 'Mr' or 'Mrs' will be able to use 'Mx'. 

They said that the changes were being introduced following close consultation with the Human Rights Campaign in order to enhance customer experience and make the booking process more inclusive.

'We are excited to share this next step as we continue to break down barriers to promote inclusion,' the spokesperson said.

A spokesperson for Airlines for America said member airlines were 'committed to making these changes to their reservation processes to account for non-binary IDs while ensuring continued compliance with US and foreign government requirements that passenger data match the identification used for travel'.

A number of states in the US have moved to introduce a non-binary 'X' designation on birth certificates and ID cards for people who do not identify as male or female. 

New Jersey introduced gender neutral birth certificates last month, following in the footsteps of California, Oregon and Washington. 

The new regulation, which came into effect on February 1, allows parents to choose non-binary as their child's gender designation while also allowing transgender people to change their gender without the need for gender reassignment surgery.