The leader of Respect, Salma Yaqoob, has resigned, blaming a breakdown in “relations of trust and collaborative working” at the top of the party.
Ms Yaqoob clashed with Respect's only MP, George Galloway, over his comments about the sexual assault allegations against WikiLeaks founder Julian Assange.
In a podcast last month, the Bradford West MP said allegations against Mr Assange did not constitute rape "as most people understand it", and amounted to no more than "bad sexual etiquette".
Mr Galloway later clarified his remarks, insisting he had always believed non-consensual sex was rape.
Ms Yaqoob condemned the original comments as "deeply disappointing and wrong" and said at the time that the complaints made by two women in Sweden should not be "belittled or dismissed". Mr Assange has denied the allegations.
The Respect leader announced her resignation "with deep regret" in a message on the party website last night.
"The last few weeks have been extremely difficult for everyone in the party," said Ms Yaqoob. "I feel necessary relations of trust and collaborative working have unfortunately broken down. I have no wish to prolong those difficulties, and indeed hope that they may now be drawn to a close.
"I remain committed to the principles and values that led me to help found Respect. The policies we have fought for need to be voiced as loud as ever in opposition to a political establishment that remains out of touch with working people.
"I would like to thank everyone in the party for their support over the years. I wish everyone the very best for the future and in those common struggles for peace, justice and equality that I am sure we will all continue to be involved in."
Respect national secretary Chris Chilvers said: "While we are obviously very sorry that Salma has decided to leave Respect, we would like to thank her for the great contribution she has made to Respect over the last decade. We look forward to working with Salma in the future in pursuit of our shared values and objectives."
Ms Yaqoob served as a Respect councillor on Birmingham City Council from 2006-11 and fought the Birmingham Hall Green constituency for the party in 2010, securing second place.