The leader of Burnley Council and 10 other councillors have resigned from the Labour Party over Sir Keir Starmer's decision not to push for a ceasefire in Gaza, Sky News understands.
Afrasiab Anwar, who has been in the party for 10 years, said it had been a "really difficult decision" to leave Labour.
He was among those calling for Sir Keir to step down on Thursday.
The 11 councillors described their memberships as "untenable" given the leadership's refusal to demand a ceasefire in the Middle East.
In a statement, they said Sir Keir had indicated he "does not value the voice of the grassroots of the party" - citing remarks the opposition leader made following a speech on Friday in the North East.
Sir Keir, who has come under internal pressure for Labour to demand a cessation of hostilities, told reporters his focus was on stopping the suffering in Gaza, not on the "individual positions" of party members.
Labour has backed the UK government's stance of calling for a pause in the fighting to allow humanitarian aid and medical treatments to reach Palestinians in Gaza.
However, Mr Anwar said the position was "nonsensical" and did not capture the strength of feeling in his Lancashire town, along with communities elsewhere in Britain, about the war.
"We just can't stand by watching and being part of a party that is not speaking out, or at the very least calling for a ceasefire," Mr Anwar said.
"Instead of talking of peace - all of our world leaders, including the leader of the Labour Party, are talking about humanitarian pauses. It's just nonsensical.
"I just don't think the message is getting through in terms of how our communities, right across the board, are feeling about this."
He said the group had tried "everything we could by working within the party".
Mr Anwar added he wanted Labour to "come back to its core values of fighting for social justice".
In a statement on Sunday, the group said: "It has become apparent that Keir Starmer and the leadership either cannot or will not heed our concerns or acknowledge the sentiments within our communities."
It added: "In response to our calls for him to resign he responded that the individual concerns of members are not his focus, further illustrating that he does not value the voice of the grassroots of the party."
The leader of Pendle Council, which is also in Lancashire, also called on Sir Keir to resign on Thursday.
It came after senior Labour figures also broke ranks to challenge Sir Keir's stance - including London mayor Sadiq Khan, Scottish Labour leader Anas Sarwar, and Greater Manchester mayor Andy Burnham.
A Labour spokesman said: "Labour fully understands calls for a ceasefire.
"Everybody wants to see an end to this cycle of violence and suffering, we need to see hostages released and aid getting to those most in need.
"But a ceasefire now will only freeze this conflict and would leave hostages in Gaza and Hamas with the infrastructure and capability to carry out the sort of attack we saw on October 7.
"International law must be followed at all times and innocent civilians must be protected.
"Labour is calling for humanitarian pauses in the fighting.
"This is the best and most realistic way to address the humanitarian emergency in Gaza and is a position shared by our major allies, in the US and the EU."