Union members on Labour's National Executive are set to call for unity and confirm their support Jeremy Corbyn's leadership, sources said.
A statement is expected later amid union anger at the moves by members of the shadow cabinet against the Labour leader.
Jennie Formby, one of Unite's representatives on the National Executive Committee (NEC) said it was "nonsense " to blame Mr Corbyn for the referendum result and said Labour MPs should show unity.
Gloria De Piero has reportedly resigned from her position in Labour's shadow cabinet.
The shadow minister for young people and voter registration joins former health secretary Heidi Alexander in stepping down.
Shadow foreign secretary Hilary Benn was sacked by leader Jeremy Corbyn overnight.
So far Hilary Benn, Heidi Alexander and Gloria Del Piero have all resigned from the Shadow Cabinet.
Heidi Alexander has spoken about her difficult decision to resign as shadow health secretary.
She told ITV's Peston on Sunday "the only decent and honourable thing to do was to resign" as she no longer had faith in Jeremy Corbyn's leadership.
The MP said she asked herself could she "hand on heart" say that she felt that "Jeremy was the best person to be leading the Labour Party in developing the answers the country is demanding".
"And I didn't feel I could so that," Ms Alexander added.
When asked if she had any sense of any other potential cabinet resignations amid mass speculation that up to half of Corbyn's top team could be about to quit, Ms Alexander said "there are a few people who feel similarly to me".
Ed Balls said Labour needs a new leader as he accused Jeremy Corbyn of alienating "both sides" in the EU debate.
He told Peston on Sunday that the party "needs a change" to show it has a vision for the future in a general election that could take place within months.
"At a time when you have a big divide between urban Britain which wanted in and heartlands an rural Britain that wanted out, Jeremy Corbyn has actually managed to alienate both sides," he said.
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John McDonnell insisted he will "never" stand for the Labour leadership as he said the party should stand by current head Jeremy Corbyn.
Labour MPs were being canvassed over their support for the shadow chancellor in a potential leadership takeover, ITV News understands.
However Mr McDonnell said he had no desire to take over the top job in an interview with the BBC's Sunday Politics programme, adding: "Jeremy's not going anywhere".
I will never stand for the leadership of the Labour Party. If Jeremy has to stand for another leadership election, I will chair his campaign and I think the Labour Party members will elect him again.
Labour MPs are being asked to support shadow chancellor John McDonnell in a potential leadership takeover, ITV News understands.
A source within the party said shadow chief secretary to the Treasury, Seema Malhotra, has been ringing around to try to canvas support.
Reports suggest she may be among the wave of resignations by shadow cabinet members expected today.
Shadow chancellor John McDonnell has insisted Jeremy Corbyn is "not going anywhere" despite the shadow cabinet being rocked by resignations.
Hilary Benn said he will not be standing for Labour leadership in any upcoming contest.
The former shadow foreign secretary was sacked in a late-night phone call with Jeremy Corbyn last night after reports he was coordinating an internal coup.
Today he said he had no ambitions to take his place and had acted because he believed Mr Corbyn was "not succeeding", in an interview on the Andrew Marr Show.
He is a is a good and decent man but he is not a leader - and that is the problem.
I am not going to be a candidate to be a leader of the labour party and I did not take this decision because I wanted to, I took this decision because I thought it was the right thing to do.
Mr Benn today said it would be "right" for Mr Corbyn to resign as he faces a motion of no confidence amid widespread revolt from MPs with reports that ten cabinet members are set to quit today.
"I think the right thing for him to do would be to take that decision, but that is a decision for him to take," he said.
Former shadow foreign secretary Hilary Benn has spoken to ITV News after his sacking by Labour leader Jeremy Corbyn.
He said: "At this critical time for our country following the EU referendum result we need strong and effective leadership of the Labour Party.
"I told Jeremy Corbyn last night that I no longer had confidence in his leadership and he dismissed me from the shadow cabinet."
He added: "I want to thank him for giving me the chance to serve our party."