Jordi Casamitjana said he was sacked by the League Against Cruel Sports after raising concerns that its pension fund was being invested into companies involved in animal testing.
He claims he was unfairly disciplined for making this disclosure and that the decision to dismiss him was because of his philosophical belief in ethical veganism.
Dietary vegans and ethical vegans both eat a plant-based diet, but ethical vegans also try to exclude all forms of animal exploitation including not wearing clothing made of wool or leather and not using products tested on animals.
For a belief to be protected under the Act, it must meet a series of tests including being worthy of respect in a democratic society, not being incompatible with human dignity and not conflicting with fundamental rights of others.
Slater and Gordon solicitor Peter Daly, who is acting for Mr Casamitjana, said "ethical veganism is a philosophical belief held by a significant portion of the population in the UK and around the world".
"This case, if successful, will establish that the belief entitles ethical vegans to protection from discrimination. The case we have prepared sets out how the belief in principle, and how Jordi's particular interpretations of it, meet the required legal test," Mr Daly said in a statement.
When he initially brought the case, Mr Casamitjana said the hearing was not primarily about his dismissal, but about establishing ethical veganism as a philosophical belief.
"Although the manner in which I was dismissed was intensely distressing for me, some good may come of it if I am able to establish this valuable protection for all ethical vegans.
"If we are successful in that hearing, we will then proceed to a hearing on the specifics of my dismissal."
But the League Against Cruel Sports said it sacked Mr Casamitjana for "gross misconduct".
In a statement to the BBC, it said: "The League Against Cruel Sports is an inclusive employer, and as this is a hearing to decide whether veganism should be a protected status, something which the league does not contest, it would be inappropriate for us to comment further."
Thursday's tribunal session will be an admin day, with the hearing due to open on Friday.