Shadow chancellor John McDonnell has accused Labour leadership hopeful Owen Smith of trying to "blackmail" the party by predicting a split if Jeremy Corbyn is re-elected.

Ahead of the first leadership debate of the contest, Mr Smith warned that Labour could "bust apart and disappear" if his rival wins the election on September 24.

Mr McDonnell insisted that Labour will unite behind whoever wins the contest and urged the Pontypridd MP - who he described as "one of the most ambitious career politicians I have met" - to end talk of division by declaring he would serve under Mr Corbyn.

The row came as a YouGov poll suggested Labour support would be reduced to around 20% if the party split - with the breakaway group performing even worse.

Mr McDonnell told Sky News: "We shouldn't be going into a leadership election saying, `Vote for me or some of my supporters will split the party'. People don't want to be blackmailed in that way. That's not right.

"Owen Smith could kill this off altogether by saying, whatever the result of the election, if Jeremy Corbyn does get re-elected he will serve under Jeremy Corbyn, and saying to his supporters who are talking up this split to stop it immediately, and let's work together."

Asked why he did not want Mr Smith as leader, Mr McDonnell said: "Owen is a great guy, I like him a lot. He is one of the most ambitious career politicians I have met. I think we have moved on from that style of politics. The style of politics people want and that attracts them to Jeremy Corbyn is, 'what you see is what you get - principled, honest, straightforward'."

Mr McDonnell refused to rule out claims by Mr Smith that when asked if he was prepared to see the party split, he ''shrugged his shoulders and said, 'if that's what it takes'''.

He told BBC Radio 4's Today programme: "I don't think I did say that. I don't recall that."

The response was markedly watered down from Mr McDonnell's previous claim that the suggestion was ''complete rubbish''.

Mr McDonnell said there had been a "pressurised" discussion with a group of MPs and he had "immediately" put out a statement saying he did not support a split.

"I have given my whole life to this party," he added. "There is no way I am ever going to allow this party to split and it won't."

Mr Corbyn is to unveil his blueprint to "rebuild" Britain before going head-to-head with Mr Smith in the first showdown of the leadership campaign in Cardiff.

The incumbent leader is expected to outline 10 pledges to mend Britain's "broken" economic model and deliver "richer" lives for all.

In a speech in Dagenham, east London, Mr Corbyn is expected to say: "There is immense potential in the skills and talents of our people and huge opportunities ahead of us in science, technology and culture.

"We could all be living richer lives in a sustainable, more prosperous and more caring society.

"Labour will pledge to rebuild and transform Britain with a bold £500 billion programme of investment.

"We will deliver the transport and communications infrastructure a modern economy needs, not just in those places where the economy already works but right across the whole country.

"We will back this up with new institutions able to deliver the investment our communities so desperately need.

"A new £250 billion national investment bank, backed up with a network of regional development banks, will put power to determine their own futures back into the hands of those places outside of Westminster and the City of London."

Mr Corbyn has previously laid out a series of pledges aimed at ending "rip-off Britain" and tackling "injustices" in the workplace, including the banning of zero-hours contracts, increasing the living wage and repealing the controversial Trade Union Act, which places restrictions on strike ballots.

Mr Smith, who walked out of the shadow cabinet after the EU referendum, has laid out a radical programme of reform - including a pensions overhaul, changes to benefits and a higher minimum wage - which he said would amount to the "biggest boost to living standards for a generation".

Meanwhile, the decision to bar Labour members who joined after January 12 from voting in the election is being challenged in the High Court, in a crowd-funded case brought by some of the estimated 130,000 people who have been excluded.