Green councillor for Brighton, Christina Summers, found out on Monday that her appeal against her dismissal had been rejected.
The party’s Brighton group of councillors decided to remove Summers after she voted against same-sex marriage at a council meeting in July.
She is considering whether to mount a legal challenge against her dismissal on the grounds that the Party’s failure to accommodate her Christian views on marriage breaches her human rights.
Ms Summers said: “I am terribly disappointed at the appeal panel’s decision, not just for myself but for others who share my views on marriage and on free speech.
“The Greens have always given the impression that they are tolerant enough to accept genuine diversity of opinion. Their decision and their statement today which, as always, dodges this substantive issue indicates, in the end, that they are not.
“I would not wish the treatment I have received on anybody.
“I am now consulting with lawyers over this fundamental breach of a human right.”