The Labour Party lost 2.6m voters between the 2017 and 2019 general elections, a major internal report has found.
Around 750,000 former supporters switched to the Lib Dems while a similar number went to pro-Brexit parties, primarily the Tories.
However, senior Labour figures claimed a further 1.2m Labour voters from 2017 decided just to stay at home.
Party strategists say that as turnout overall only went down slightly it was a “reasonable assumption” that non-voters who had turned out in 2016 to back Brexit, had come out again to get the job done.
The official 44-page Labour report, circulated to the party’s ruling National Executive Committee and seen by the Mirror, raised concerns over target seats picked by the leadership.
At the start of the campaign, 96 constituencies were singled out for extra leaflets, online ads and newspaper promotions.
The report said: “As the campaign progressed it became clear that we were not closing the gap between ourselves and the Tories as rapidly as we had in 2017 [with] the biggest problem in Leave areas”.
A further 37 seats were then identified as vulnerable and added to the list for extra support – spreading resources even thinner.
It comes as Labour’s membership surged to more than 580,000 – its highest number ever – after its general election defeat.
A senior Labour source said: “It was already by far the biggest party in Britain. It is now even bigger – as big as it has ever been.”
New members will vote for Jeremy Corbyn’s successor who will be tasked with restoring the party from its worst defeat since 1935.
While it is unclear which candidate they will back, few party insiders believe they will benefit Rebecca Long-Bailey the current leadership’s preferred candidate.
The surge of around 20% will also give Labour a much-needed boost to its, which party sources told the Mirror were in a “dire” state.
They said Labour had spent around £8m at the election, way below the £19m national limit for the six-week “short campaign”.
The extra “third party” funding from the unions and Momentum would come on top of that.
Before the election, party chairman Ian Lavery claimed Labour had “never been more ready for an election”.
The 44-page report seen by the Mirror found that Labour lost most support to the Tories among low-income older voters and less well-off families, both traditionally seen as a key party demographic.
It lost ground in “blue collar” areas with industrial pasts, low level of educational qualifications and ageing populations.
For the first time, the party’s vote share in these seats was significantly lower than in big cities and their suburbs.
They concluded that it’s support was in “long-term decline” in these largely working class areas, even though they still constitute half of Labour seats, while the Tories had gained ground.
The internal paper also revealed that of the 20 constituencies where Labour had the most “contacts” with voters, it only gained one – Putney – while losing Kensington and hanging onto five more. All the rest remained Tory.
However, party activists made an astonishing 2.4m “contacts” on the doorstep, underlining the strength of local party machines.
Labour sources told the Mirror that the majority of contributions at an NEC “away day” on Tuesday to discuss the election result were “head-in-the sand” supportive of Mr Corbyn, who gave a “long and rambling” contribution but failed to apologise for his leadership.
Verbal reports were given by Mr Lavery and elections chief Andrew Gwynne.
One source at the meeting said Mr Lavery blamed media attacks for Mr Corbyn’s poor leadership ratings.
He criticised some MPs and shadow cabinet members for “indiscipline” which he said undermined the party’s efforts.
The insider said: “It was like a parallel universe. You’d have thought we were the slickest, best run party and had actually won.”
However, Mr Gwynne had apologised for the “devastating” result and told colleagues: “If we don’t learn the right lessons of our defeat under the new leadership then we’ll be out of power for longer than just four years”.
And he admitted that Mr Corbyn’s leadership had “drawn on” Labour’s support.
But a second Labour party report, published by the FT, blamed Brexit – while largely absolving Mr Corbyn from blame.
The post-mortem found it would be “unrealistic” not to say its policy to hold a second Brexit referendum played a “decisive” role.
While the authors accepted there were “negative views” of the outgoing leader they did not lay direct blame at his door.
Far from being a weak or divisive leader, they concluded instead that he was the victim of four years of unrelenting attacks on his character. This was an “assault without precedent in modern politics”, they said.
But they did admit that a glut of policies had been confusing for the public – although it defended their radicalism.
NEC members were shown a third report on the party’s community organising efforts by Karie Murphy, Mr Corbyn’s ex-chief of staff who led the day-to-day campaign.
The data showed the extent to which “contact rates” with voters did not translate into Labour seizing any seats – apart from Putney.
In Colne Valley in the so-called “red wall” the party made contact with 12,878 voters – up from 5,957 in 2017 – but the seat still switched from Labour to Tory.
One party source described Ms Murphy’s report to the Mirror as “utter whitewash”.
“I fear it’s an excuse to restructure the party before the new leadership takes over. The community organisers were useless in many cases.”
A group called Labour Together – including former party leader Ed Miliband – is carrying out a further review of the election result and is unlikely to be generous to Mr Corbyn.