Former Labour chair Ian Lavery has accused Keir Starmer of carrying out a "purge of left wing members" and breaking a promise to unify the party.
Mr Lavery said he would do "everything I can" to ensure Labour succeeds.
The Wansbeck MP, who was party chair under former leader Jeremy Corbyn, was responding to Sir Keir's decision to sack Rebecca Long-Bailey as shadow education secretary.
Mr Lavery said: "I'm very concerned at what I see as disproportionate action against one of the most talented loyal female members of Parliament in the commons. Rebecca Long-Bailey could have been leader of the party under different circumstances.
"Sir Keir stood as a unifying candidate in the elections expressing the need for a united party and within a matter of months has shown the very opposite. The purge of left wing members continues at a pace.
"Many people from our region seem to think that Keir Starmer has a problem with Northern voices. I don't subscribe to that view.
"Political parties only win elections when wholly united with a clear joined up message. At the moment we're far from that with cracks appearing on a daily basis.
"Like others I want the party to succeed and will do everything I can to make that happen. Our communities have been neglected for far too long left behind and ignored. This cannot continue. Attacking each other does not help in anyway."
Mr Lavery has also released a video in which he urges left-wing Labour members to stay in the party.
Blog Skwawkbox, which has close links with left-wingers in the Labour Party, has reported that left-wing MPs are compiling a list of supporters for a potential leadership challenge next year, after the 2021 local elections.
Mr Lavery is a member of the Socialist Campaign Group of Labour MPs, which on Friday held a meeting with Sir Keir via video-conferencing, to express concern at Ms Long-Bailey's sacking.
Other members include Easington MP Grahame Morris and Gateshead MP Ian Mearns, as well as Jeremy Corbyn, former shadow chancellor John McDonnell and former Shadow Home Secretary Diane Abbott.
Ms Long-Bailey was the last of the MPs regarded as being on the left of the party to hold a shadow cabinet post.
"She was sacked after Tweeting a link to an interview with actress Maxine Peake, in which Ms Peake suggested Israeli secret services had taught "the tactics used by the police in America, kneeling on George Floyd’s neck." Ms Peake has since said that she accepts this is not correct.
Ms Long-Bailey was accused of spreading an anti-semitic conspiracy theory. She stated that she did not agree with everything said in the interview, but was nonetheless sacked from the shadow cabinet.
A spokesman for Mr Starmer said: "The article Rebecca shared earlier today contained an antisemitic conspiracy theory.
"As Leader of the Labour Party, Keir has been clear that restoring trust with the Jewish community is a number one priority. Antisemitism takes many different forms and it is important that we all are vigilant against it."
