Jeremy Corbyn’s “disastrous” performance is pushing Labour towards its worst general election since the 1930s, a leading member of his own team has warned. The top frontbencher told The Independent that if the devastating Copeland by-election result were mirrored elsewhere, Labour would be left with just 150 seats in the House of Commons.

But with Mr Corbyn resisting calls to quit, the senior figure also accepted there was no way of preventing him leading the party into the 2020 election, adding: “We are just going to go off the cliff.”

Another leading Labour figure, Angela Smith, the shadow leader of the Lords, went on record to brand the Copeland defeat “devastating”, adding that Mr Corbyn would have to think “long and hard” after handing the Tories the first by-election gain for a sitting government in 35 years.

Other frontbenchers in the Commons also privately stated the result in Copeland was far worse than Mr Corbyn and his closest allies publicly admit, with one supportive shadow cabinet member even saying: “It was a really bad night for us.” They also dismissed the idea that the Stoke-on-Trent Central by-election win balanced out the Copeland defeat, highlighting the swing away from Labour in both seats.

With pressure mounting on Friday, one of Mr Corbyn’s key union backers, Dave Prentis, general secretary of Unison, demanded the leader “take responsibility” for what happens next.

Jeremy Corbyn ducks resignation questions after byelection humiliation

In by-elections on Thursday night, Labour held Stoke-on-Trent Central against a shambolic challenge from Ukip leader Paul Nuttall, while still suffering a 2 per cent swing to the anti-immigration party. But it was the result in Copeland, where they were hit by a 6 per cent swing to Theresa May’s Tories, that left the party reeling. The Prime Minister hailed the victory as proof that her Government “delivers for everyone across the whole country”.

The top frontbencher told The Independent: “The Labour Party should not be losing seats like Copeland, but there is nowhere to go and nothing to be done. The party is controlled by a membership that believes our leader can do no wrong.

“There is no scope for a leadership challenge, it was tried and didn’t work – we are just going to go off the cliff.”

He added: “He will take the Labour Party into a general election and own the result, but if it reflects what has happened we are going down to 150 seats.”

Not since Stanley Baldwin’s Tories wiped out Labour in October 1931, knocking it down to just 52 seats, would the party have sunk so low.

Former foreign secretary David Miliband spoke out after the party’s dismal performance in Copeland, warning that Labour’s chances of electoral success had shrunk to their lowest point in a generation.

Mr Miliband, who stood for the leadership of the party in 2010, told The Times: “I’m obviously deeply concerned that Labour is further from power than at any stage in my lifetime.”

Shadow cabinet minister Baroness Smith, elected to her post by other Labour peers rather than selected by Mr Corbyn, also said Copeland was “devastating” for Labour.

She added: “We’ve got to reflect on this. We mustn’t bury our head in the sand, this is a very bad election result for us.”

Mr Corbyn, shadow Chancellor John McDonnell and other allies of the leader took to the airwaves on Friday to ensure that the by-election win in Stoke was not forgotten and to mitigate damage from the Copeland loss.

Labour MP Cat Smith even said: “To be 15 to 18 points behind in the polls and to push the Tories to within 2,000 votes [in Copeland] is an incredible achievement.”

But another frontbencher, who supports Mr Corbyn, told The Independent: “The party is spinning it that it’s good that we defeated Ukip in Stoke, and it is, but the reality is that it was a really bad night for us because Copeland was a really bad loss.

“It can’t all be put at Jeremy’s door: the decline in vote share in Copeland has been happening for 15 years. But there has been a major shift in the demographic of the country, and it’s for the leadership of the party, yes Jeremy, but all of us in the shadow Cabinet and in senior roles, to take responsibility for dealing with that.”

On Friday, John Woodcock said the Copeland result showed the party was heading for a “catastrophic” election in 2020 with Mr Corbyn at Labour’s helm.

Labour MP David Winnick said Mr Corbyn was an “obstacle” to victory and should consider his position.

But another senior backbencher said the by-elections had exposed a deeper problem: “It’s not just about him, it’s about all the people around him too. They are getting mentioned on the doorstep as well – Diane Abbott and John McDonnell in particular.

“The whole direction he and the people around him are taking the party in, is the problem – it’s the whole crew.”

Insiders at Labour HQ said the party had run robust and competitive campaigns in both seats, with adequate funding, strong candidates and a focus on Labour’s key issues, highlighting that causes for the loss lay elsewhere.

Unison General Secretary Dave Prentis, who backed Mr Corbyn in the leadership contests, said “no one objective” could argue the two by-election results were good for Labour.

He went on: “While it was pleasing to see Ukip put in its place, Stoke should never have been in doubt and the result in Copeland was disastrous.

John McDonnell shifts blame on to Tony Blair and Peter Mandelson after Labour Copeland by-election defeat

“The blame for these results does not lie solely with Jeremy Corbyn, but he must take responsibility for what happens next.”

Gerard Coyne – the senior figure challenging Mr Corbyn's ally Len McCluskey for the top job at Unite – said the defeat represented a “meltdown”. He demanded Mr McCluskey account for spending tens of thousands of pounds in union money on keeping Mr Corbyn in office instead of prioritising members’ issues.

A spokesman from Mr McCluskey’s campaign for re-election told The Independent that Unite members would be “baffled” by any suggestion the general secretary was not putting members first.

Labour campaigns chairman Ian Lavery insisted the by-election was not a ballot on his leader’s performance, claiming on Friday afternoon that “Jeremy Corbyn is one of the most popular politicians in the country at this moment”.

Mr McDonnell, meanwhile, hit out at Tony Blair and Lord Mandelson for high-profile interventions that he argued had served to divide the party just days before the polls opened in Copeland and Stoke.