In his letter, Hussain said he had believed he was signing up to “building a political home with mass appeal” and “a force capable of challenging the rise of far-right rhetoric”.
“Regrettably, the reality I encountered has been far from this vision.
“The culture surrounding the party has become dominated by persistent infighting, factional competition and a struggle for power, position and influence rather than a shared commitment to the common good.
“Instead of openness, cooperation and outward focus, the environment has too often felt toxic, exclusionary and deeply disheartening.”
The Blackburn MP added: “I have also been deeply troubled by the way certain figures within the steering process, particularly Muslim men, have been spoken about and treated.
“At times the rhetoric used has been disturbingly similar to the very political forces the left claims to oppose. I witnessed insinuations about capability, dismissive attitudes and language that carried, at the very least, veiled prejudice.”