Jeremy Corbyn faces a coup this week by members of his shadow cabinet, led by Hilary Benn, the Observer can reveal.

It is understood that the shadow foreign secretary called fellow MPs over the weekend to suggest that he will ask Corbyn to stand down if there is significant support for a move against the leader. He has also asked shadow cabinet colleagues to join him in resigning if the Labour leader ignores that request. A spokesman for Benn declined to comment.

An overwhelming majority of the shadow cabinet now believes Corbyn should quit in the wake of millions of Labour voters ignoring their leader’s advice to vote in favour of Britain’s continued membership of the EU and amid the possibility of an early general election.

The development comes as leaked internal Labour party polling of people who voted for Labour in 2015 reveals that nearly a third (29%) would support a different party if a general election was held today.

A Labour source said: “MPs and members were worried about their prospects at the next election under Corbyn, but thought they had four years to turn things around. Now many fear they may have just four months if a snap election is called.”

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The development looks likely to be the most serious threat to Corbyn’s leadership yet, with many MPs claiming that he must be unseated by the end of the week for Labour to remain an electoral force.

On Saturday the Labour leader sought to minimise the danger during a question and answer session with the media at the end of a hastily arranged speech in central London.

Asked whether he would put himself forward in the event of a Labour leadership contest, Corbyn offered a cursory response. “Yes, I’m here,” he said.

Asked again, Corbyn dismissed the threat and referred to a petition in support of his continued leadership. “Yes, there are some people in the Labour party, and the parliamentary Labour party in particular, who probably want someone else to be the leader – I think they’ve made that abundantly clear,” he told reporters and activists.

“What I’m totally amazed by is that 140,000 people have said they do not want the party to spend the next two months debating the leadership of the party; they want the party to get on the front foot, and get out there.”

However, the Observer understands that members of the shadow cabinet have been in constant communication through the weekend because of serious concerns about the future of the Labour party.

Facebook Twitter Pinterest Hilary Benn, the shadow foreign secretary, is understood to have called fellow MPs over the weekend. Photograph: Giles Price/The Guardian

Senior sources said on Saturday that Benn had been ringing shadow cabinet colleagues over the past 24 hours asking two questions. First, sources say, he asks whether he should ask Corbyn to stand down. He is then said to ask if they will join him in resigning should Corbyn refuse.

A second shadow cabinet minister confirmed that the consensus among MPs was that Corbyn’s position was untenable – and at least half of the shadow cabinet agreed. “This is an existential threat to Labour now,” said one shadow cabinet minister.

The Labour electorate in the northern heartlands, from Sunderland to north Warwickshire, rejected Corbyn’s advice to support Remain in order to back the political message of Boris Johnson and Ukip’s Nigel Farage. “It might be too late. But we have to do something,” added the source.

Many outside the top echelon of the party are understood to agree. On Tuesday, following a meeting on Monday, the parliamentary party is set to vote anonymously on a motion of no confidence in Corbyn. In recent days key backbenchers have been coalescing support, using the secure Whatsapp service to communicate.

They now believe that they have at least 80% of the party on their side although it is understood that the chief whip, Rosie Winterton, is seeking to have the vote delayed for a week.

Corbyn critics said they feared that this would allow MPs to organise in support of a leadership bid by McDonnell, in the event of Corbyn resigning under pressure.

Allies of Corbyn and McDonnell, his long-time friend and closest ally, say that it is madness for a leadership contest to be sparked just when the Conservatives have lost a prime minister and are embroiled in their own internal chaos.

They also suggest that Corbyn was not to blame for the failure of people who have historically supported Labour to back Remain. Rather, they ascribe that to the failings of Labour governments in the past to connect with their traditional base of voters. They also claim that the remain campaign run by Jack Straw’s son, Will, was incompetent.

McDonnell told the Observer that the infighting was an unnecessary distraction. He said: “We need a Labour voice to be heard and not to be crowded out by noisy infighting. It is the responsibility of Labour politicians to remember our duty to represent the will of our party and the Labour movement to fight for working people, and not to fight each other.”

However, critics claim the Brexit vote has shown how disconnected Labour has become from much of its support and has crystallised for many how far Labour is from being in a position to win a general election.

Leaked internal Labour party polling suggested that Labour would attract nearly 3 million fewer votes than it did in the 2015 general election if one were called today.

It shows that just 71% of those who voted for Ed Miliband’s Labour party in May last year say they would vote Labour now, and this drops further – to 67% – among working and lower middle-class C2DE voters.

Some claim that the Labour northern heartlands are breaking off from the party, as they have already done in Scotland. If there is a general election soon, as it appears may be the case, Labour could be decimated, it is said.

Writing in the Observer, the former shadow education secretary, Tristram Hunt, has made public his belief that a change needs to be immediate.

“Our experiment with Corbynism has to end,” he writes. “If Labour members care about Labour voters, we need to do something about the Labour leader … I do not think Corbyn is a bad person, but he has proved utterly ineffectual.”

Stephen Kinnock, the Labour MP for Aberavon, also adds his voice to those who say that they have lost confidence in their leader. He said: “We must … have a full and frank discussion when the parliamentary Labour party meets, to look at what went wrong, and what we should learn.

“Our leader must be held accountable for the failure of the Labour In for Britain campaign, as must we all. It is vital that Labour has a seat at the top table, and critical that we have a leader who has the right experience and skills for the task at hand.

“And it is for that reason that I am supporting this motion of no confidence.”