Twenty shadow ministers say they are willing to call for him to stand down, if Alan Johnson is willing to step into the breach

Ed Miliband’s Labour party leadership was plunged into fresh crisis as senior Labour MPs revealed that at least 20 shadow ministers were on the brink of calling for him to stand down.

The frontbenchers are willing to go public with their demand if the former home secretary, Alan Johnson, indicates that he would be prepared to step into the breach, should Labour be left leaderless just months from a general election. The senior status of the potential rebels and their numbers represent another grievous blow to Miliband’s authority, after attempts by his aides to minimise the extent of dissatisfaction in the party.

The rebels are drawn from across the political spectrum within the party and say they are airing their discontent in response to attempts by Miliband’s aides to belittle the depth and reach of unhappiness among his MPs.

Three senior Labour MPs contacted the Observer on Saturday to confirm that the frontbenchers were ready to strike. One said: “It has reached critical mass now.” Another said: “There isn’t a letter [demanding Miliband’s resignation] but there could be one very quickly.” A third said: “There is a significant number of frontbenchers who are concerned about Ed’s leadership – or lack of leadership – and would be ready to support someone who is a viable candidate. Alan is that candidate. If Alan indicated he would do it, there would be a massive move”.

The development comes as an Opinium/Observer poll suggests that, for the first time, less than half of Labour voters (49%) approve of Miliband’s leadership – although the party is on 32%, still ahead of the Tories by three points.

Labour’s autumn slump. Graphic: Giulio Frigieri/Guardian

The Observer understands that senior MPs have been taking soundings through the last week about the prospect of removing Miliband just six months before the 2015 general election.

The ringleaders of the prospective coup have collected 20 names of frontbench colleagues – about one-fifth of shadow ministers – who expressed concern at the seriousness of the party’s plight and would act if Johnson offered any encouragement. They believe others would also support the move.

On Saturday, Johnson appeared to suggest he was not willing to step in, in the event of Miliband stepping down. He told the Times: “For the avoidance of doubt, I have no intention of going back to frontline politics.”

However, one leader of the rebellion said he believed that this was not a categorical rejection, and that Labour MPs would be looking for any hope of a change of heart.

It has been a harrowing week for the Labour leader, whose aides have blamed the rightwing press for exaggerating the scale of the discontent in the party in splash headlines alleging conspiracies among shadow cabinet members. Those claims have been strenuously denied, but the latest development will be a massive blow on the eve of a speech to the CBI, in which Miliband was hoping to reap the rewards of the prime minister’s troubles over the EU’s £1.7bn surcharge and the expected loss of a second seat to Ukip in Rochester and Strood at the byelection on 20 November.

A senior member of the prospective coup told the Observer: “There are 20 frontbenchers who are actively considering what is best to do. They are from all areas of the party, bar the hard left.”

The source added that the party would not want a leadership contest so close to the general election. Candidates such as shadow home secretary Yvette Cooper, shadow health secretary Andy Burnham, and shadow business secretary Chuka Umunna would all be contentious and were not regarded as viable.

Instead, the shadow ministers want Johnson to be crowned as leader without a contest. A Labour MP said: “What people are pondering is, if people were to ask Ed to go, then what’s the alternative? I don’t think there is any appetite for a leadership contest. There would have to be a candidate that everyone can support. If there was a sense that there was an alternative, then things could move pretty quickly.”

Another source said: “It’s not that we haven’t been concerned for a long time. But there has been no improvement in his performance and there doesn’t seem any sign that there will be”.

On Monday Miliband will warn that David Cameron’s threat to leave the European Union in less than three years’ time represents a clear and present danger to Britain’s future prosperity and national interest.

He is expected to say: “We have seen the rise of forces that want to drag us out of European Union and close us off from the world. These false solutions would be terrible for our country and terrible for your business, for those who work for you, and for Britain itself.

“Leaving the single market and stepping away from a trading block that strengthens Britain’s ability to work with the new economies, like Brazil, India and China, would be a disaster for our country. It would risk businesses billions of pounds in lost profits. It would risk millions of jobs. It would make Britain weaker, not stronger, in the world.

“There are some people in our country who advocate exit from the EU. There are others who flirt with it, thinking they can do so without consequence. Both are equally dangerous.

“Giving succour to the argument that the real answer is leaving the EU, or contemplating it, simply drags us closer to exit.”

More on the Labour leadership

Plotters circle in the shadows as embattled Ed Miliband strikes back

Ed Miliband at bay: the hack pack scents the blood of vulnerable prey

The Observer view on Miliband and his leadership

If Labour is stuck with Ed Miliband, here’s a way out

Comments for this article will be switched on by staff on Sunday morning.

