Does an employee need to undergo medical treatment to reassign his or her gender to be protected against discrimination?

No, the definition of gender reassignment under the Equality Act 2010 does not require a person to be under medical supervision in order to come within it. The definition covers people who are "proposing to undergo" a process for the purposes of reassigning their sex. The Equality Act 2010: code of practice on employment advises that a person does not have to have reached an irrevocable decision to reassign his or her gender. It states that people who have started a gender reassignment process but have withdrawn still have the protected characteristic because they have undergone part of a process of gender reassignment.