A victory for Jeremy Corbyn in the Labour leadership race will reduce the party's chances of winning the next election, a poll for The Independent on Sunday reveals.

As ballot papers arrive for more than 600,000 members and supporters, the wider electorate thinks Mr Corbyn stands the least chance of returning Labour to power in 2020, the ComRes poll shows. In a dramatic twist, David Miliband, defeated by his brother in 2010, would stand the most chance of winning for Labour.

With Mr Corbyn still the odds-on favourite to win on 12 September, The IoS has learnt that his team has already begun preparing his shadow cabinet. He is planning to make John McDonnell, the leader of the Socialist Campaign Group who stood against challenged Gordon Brown in 2007, shadow Chancellor, a source revealed. Mr Corbyn’s aides have also sounded out Tom Watson, the front-runner for deputy leader, for a party management role.

As many as nine members of Labour’s current Shadow Cabinet are ready to quit if the Islington North MP wins, and many MPs are preparing to join the Labour for the Common Good group, set up by Chuka Umunna and Tristram Hunt, which aims to bring together the soft left, old right, Brownites and Blairites as a moderate pressure group in anticipation of a Corbyn victory.

Liz Kendall, who is fourth in the polls, said her supporters could “hold their heads up and be proud” of what they believed in, despite a campaign of personal abuse against the candidate and anyone who backed her. Ms Kendall said a Labour councillor had told her he had been threatened with deselection if he voted for her.

In what could be seen as a valedictory message, she told her supporters via this newspaper: “Hold your heads up high and be proud of what you believe in. It is your party, too. Don’t be intimidated into thinking you shouldn’t express those views for fear of being attacked. That is not the kind of party we are. Just be proud, it is our party and don’t let anyone tell you otherwise.”

The MP for Leicester West added: “What is really heartening is the many party members who believe in having a modern, credible Labour Party and are desperate for us to talk to the public and not just ourselves, and who want to see a leader who can fight the Tories and get us elected.

“There are many tolerant, sensible Labour Party members who need a voice and need someone to represent their views and that is what I am doing.”

The ComRes poll finds 31 per cent of voters think that Mr Corbyn would worsen Labour’s chances if he became leader, as against 21 per cent who say he would improve them, an overall score of -10. Out of the leadership candidates, Andy Burnham is best placed to improve Labour’s chances, on +5, followed by Yvette Cooper on -3 and Ms Kendall on -6. But the potential leader with the best rating is David Miliband, who quit British politics two years ago, with an overall score of +11.

The poll finds that more than twice as many British adults think that Mr Corbyn as Prime Minister would make the state of the British economy worse rather than better (36 per cent versus 14 per cent); while three times as many think he would make Britain’s standing around the world worse rather than better (37 per cent to 11 per cent).

The only aspect that voters think would be improved under a Corbyn premiership is the railways, with 23 per cent thinking they would be better compared to with 22 per cent who think they would be worse.

The poll also gives the Conservatives an 11-point lead over Labour on general election voting intention, at with 40 per cent to 29 per cent.

Separately, the results of a survey, based on phone-bank data for one campaign and seen by The IoS, suggest that Ms Cooper is picking up support from undecided voters. The figures put Mr Corbyn on 40 per cent, with Ms Cooper second on 17 per cent, and Andy Burnham third on 15 per cent. Ms Kendall has only 7 per cent of support. Some 21 per cent said they were undecided, but the figures suggest that Mr Burnham is losing votes to Mr Corbyn and some of the undecided are switching to Ms Cooper.

Ms Cooper will today launch a plan to fight “rip-off” payday lenders by funding more affordable loans on the high street.

A one-off levy on payday lenders will fund an Affordable Credit Trust, which is intended to raise hundreds of millions of pounds.

The trust will support credit unions in providing affordable loans, with capped and affordable interest rates, as well as allowing people to exchange their high-cost payday loans for more affordable credit.

Ms Cooper will also vow to toughen up the overall cap on the price of payday loans introduced by the Government earlier this year, to stop payday lenders charging customers more after the one-off levy.

She said last night: “Too many of these payday lenders are still ripping people off and profiting from desperation. It’s time those lenders paid for an affordable alternative. That’s why I think they should pay to support credit unions that can help people instead.”

In an article for The IoS, Mr Watson set out his vision for his deputy leadership, which he says will harness the enthusiasm showed shown by the around 400,000 people who have joined the party either as members or registered supporters since the election, regardless of who is elected leader. He writes: “We may be witnessing a new chapter in British political history. The Labour Party now has more than 600,000 members and registered supporters – a base that no other party can compete with. It is a mass movement that could sweep Labour back into power in 2020. But for that to happen, we need to harness the energy and enthusiasm of existing and new members alike, to build a real political movement with a strong base in every community.”

As if elected deputy leader, he would introduce a bursary scheme for parliamentary candidates who needed financial assistance in order to attract more would-be MPs from manual and clerical backgrounds.

He would also appoint Gloria De Piero, the shadow equalities minister, to chair an inquiry into increasing how to best increase the number of women Labour councillors.

Meanwhile, the Labour for the Common Good group will hold its first meeting on 8 September, four days before the new leader is announced. In an email from Mr Umunna and Mr Hunt to fellow MPs, and leaked to this newspaper, the pair say the group will aim to be “as inclusive as possible” and “explore how we translate Labour values into reality, in an era very different to that which began in 1994 and under Labour leaderships before that…

“The leadership contest has been tortuous, but has exposed the fact that we have failed to do sufficient political and intellectual mobilisation in this regard. There is now an urgent need for moderates in our party to do so, if we are to inspire hope in the party and the country again.”

John McDonnell: Corbyn's right-hand man

John McDonnell may not be a household name just yet but in a little over a month from now he could be shadow Chancellor and facing George Osborne at the Dispatch Box.

Like Jeremy Corbyn, the MP for Hayes and Harlington is a veteran left-winger. The Liverpool-born chair of the Socialist Campaign Group of MPs, he stood against Gordon Brown for the Labour leadership back in 2007 but did not gain enough nominations from MPs to get onto the ballot and prevent a coronation.

In the 1980s, Mr McDonnell was chairman of finance and deputy to Ken Livingstone on the Greater London Council (GLC) but the pair fell out over rate caps imposed by the Thatcher government and Mr McDonnell was removed as deputy in 1985. Mr McDonnellHe had wanted to follow Liverpool council, then run by the Militant Tendency, to rebel against rate capping.

After the GLC’s abolition of the GLC in 1987, Mr McDonnell held positions in local government before becoming an MP in 1997. He is a fierce opponent of expansion at Heathrow, because his constituency is one of the places most affected by noise and environmental pollution at from the airport.