Robin Harper has resigned his life membership of the Scottish Green party as he believes it has “lost the plot”.
Mr Harper, who was the UK’s first Green parliamentarian, quit in a letter to co-leader Patrick Harvie obtained by the Times newspaper.
He said he was concerned by the the party’s pro-independence stance and its position on transgender rights.
Describing himself as “politically left of centre” he wrote that he has been troubled by the party “moving into the gap left by the socialists”.
Mr Harper said the question of independence “came into sharp focus for me” when he was invited to join the pro-UK think tank Our Scottish Future, which was started by Gordon Brown and with which Mr Harper is now active.
He added: “The video I recorded for them, which you will find online, sums up my intention to fight any attempt to needlessly destroy the United Kingdom.