Labour will need more than "one more push" on its general election strategy if it is to capitalise on growing support and form a majority government after the next vote, a think tank has warned.
Policy Network said Jeremy Corbyn's party can only win a Commons majority at the next election if it appeals to lower middle income voters in the "C2" social grade - the "just about managing" targeted by Theresa May and the Tories.
This is because the party needs to win 64 seats, more than twice its net gain on June 8, and almost two-thirds of these constituencies have more C2 voters than upper and middle class people.
It is those skilled working class voters who are most sceptical and have a less favourable view of the Labour Party, the research shows.
The think tank, which counts Tony Blair's right-hand man Lord Mandelson as its president, said Labour did best among the highest and very lowest earners in Britain, accounting for victories in places as diverse as Knowsley and Bristol West.
But middle-income earners with salaries from £21,000 to £34,000 are nine points less likely to vote Labour than someone earning less than £14,000 a year.
The report also found that, contrary to suggestions that students and young first-time voters drove Labour's above-expectations performance at the election, polls suggest 25 to 44-year-olds, particularly graduates, switched in large numbers to the party and made the decisive impact.
Having a degree increased the chance of a voter backing Labour by nine points, while the Tories led among over-35 year-old non graduates.
In a warning over Mr Corbyn's Brexit stance, the think tank found that Labour benefited more than the Liberal Democrats from a backlash of voters who backed Remain in the EU referendum.
But following a rebellion from 50 of his MPs over single market membership, it would appear the Labour leader faces a challenge keeping these voters onside given his commitment to leave the bloc.
The think tank also found that Labour is seen as the party of "equality", but more voters want it to embody "fairness", particularly those less likely to support Labour at the election.
A perception of economic incompetence and the view that the party will not negotiate a good Brexit deal represent Labour's biggest vulnerabilities, while the party is strongest on dealing with falling living standards, everyday community concerns and public services.
Labour now has two choices - either follow a "Bernie Sanders" approach to boost support among those who backed it on June 8, or the "Clement Attlee" strategy of building a coalition of voters across class divides and winning over those least likely to support the party, the think tank said.
Only the second approach would deliver the majority required for Mr Corbyn to achieve his goal of transforming Britain, as the post-war Attlee government did.
Report co-authors Patrick Diamond and Charlie Cadywould said: "A 'one more push' approach at the next election may be enough to allow Labour to cobble together an unstable minority government, but much more is needed to win an outright majority even of one, let alone a comprehensive victory which would produce a two or three-term government able to deliver radical reform.
"Put simply, for Labour to win a majority, it can't forget about the lower middle classes, and this polling shows it has a lot more work to do before it wins sufficient support from making ends meet Britain."
:: For the report, Populus surveyed 2,511 people online in Great Britain from June 2 to 5, including boosters of 2015 and 2017 Labour voters to ensure a robust sample size for detailed analysis. The overall results were weighted by 2017 general election result and down-weighted the 2015 Labour sample.
Policy Network's analysis of the data includes a multivariate logistic regression model, which allows researchers to isolate the effect of individual factors on the probability of voting Labour, controlling for other variables.