Salma Yaqoob has quit as leader of the Respect Party after its standard-bearer and founder George Galloway caused outrage with his comments about rape.
In a statement published on the Respect Party website on Tuesday, Yaqoob said "necessary relations of trust and collaborative working have unfortunately broken down".
"I remain committed to the principles and values that led me to help found Respect," she said.
"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."
In August Galloway said the crime WikiLeaks founder Julian Assange was accused of was not rape and was simply guilty of "bad sexual etiquette".
The Bradford West MP's comments were condemned by Rape Crisis as "offensive and deeply concerning".
At the time Yaqoob said Galloway's comments on what constitutes rape were "deeply disappointing and wrong".
"There are many political issues entwined in the case of Julian Assange. These issues cannot be used to diminish in any way the seriousness of any allegations against him," she said.
Yaqoob contested the Birmingham Hall Green constituency at the 2010 election, coming second to the Labour Party's Roger Godsiff.
In response to her resignation, the party said it was "sorry" that she had decided to leave and thanked her for her "great contribution".
"We look forward to working with Salma in the future in pursuit of our shared values and objectives," it said.