London: Lord Mandelson tried to persuade the three mainstream Labour leadership candidates to quit en masse in an attempt to stop Jeremy Corbyn, the Left-wing favourite.

The peer, one of the architects of New Labour, is understood to have hoped the party would suspend the contest.

He had to back down when informed by officials that the withdrawal of Liz Kendall, Yvette Cooper and Andy Burnham would mean certain victory for Corbyn.

It also emerged on Sunday night that Kendall urged Cooper to stand down because Burnham was the only candidate of the three who could win - but Cooper refused.

Cooper has played down the reports on Monday, as voters begin to receive ballot papers.

Cooper said she understood there was a “view” the race should be stopped.

But she told the Today programme she did not think this was “right” and she had not spoken to Lord Mandelson.

The claims lay bare the desperation of the Labour hierarchy to stop Corbyn from succeeding Ed Miliband as leader in less than four weeks.

The disclosures were made as Gordon Brown, the former Labour prime minister, effectively begged hundreds of thousands of party members and supporters not to vote for Corbyn when they receive their ballot papers in the next 48 hours.

It emerged that four unions - including Unite and the RMT train drivers’ union - have donated or lent nearly £100,000 to Corbyn’s so-called “crowd sourced” leadership campaign.

Burnham prepared to claim in a speech Monday that he is the “only candidate” in the leadership campaign who can unite Labour and “lay the foundations for a Labour victory in 2020”.

Lord Mandelson privately appealed last week to the Kendall, Cooper and Burnham camps to quit the contest before ballot papers were sent out, according to sources. One said: “Lord Mandelson and other Blairites were saying - this is a disgrace, let’s get this thing pulled. But it was not going to happen.”

There were also claims that Kendall approached Cooper to ask whether she would quit to back Burnham and help him overhaul Corbyn’s lead. Cooper refused.

A senior source from Kendall’s camp said that its polling data showed Burnham was best placed of the three candidates to beat Corbyn. Sources close to Kendall and Cooper dismissed claims that they had discussed quitting. Mandelson did not reply to requests for comment. Labour declined to comment. The divisions, published in a column by Dan Hodges on the Telegraph’s website, were disclosed as Brown stepped into the leadership campaign with a thinly veiled attack on Corbyn’s ambitions to be leader.

In a 50-minute speech at London’s South Bank centre, Brown name-checked Labour leaders from Keir Hardie to Michael Foot as he gave warning that the party would be powerless to help the poorest and most vulnerable unless it could win a general election.

Acknowledging that the party was “grieving” after its election defeat, he said: “There is one thing worse than having broken hearts, it is powerlessness.”

Labour had to be “electable”, Brown said, adding: “We cannot deliver in government without power, we can deliver principles only when we have power.”

Brown did not refer to any of the four leadership candidates by name during his speech. Afterwards he refused to comment on who he would vote for in the contest when challenged directly by The Daily Telegraph. But in a clear indication that he was concerned about a victory for Corbyn, he singled out some of the potential foreign allies for whom the would-be leader has expressed sympathy. He said the rise in popularity of non-establishment politics was a response to the insecurity created by globalisation.

Brown said: “If we are going to solve the problems of both the global economy, global finance, global climate change, if we are going to solve the problems of global inequality and poverty, we will need a level of global co-operation to match our national endeavours that is higher. “If our global alliances are going to be alliances with Hezbollah and Hamas and Hugo Chavez’s Venezuela and Vladimir Putin’s Russia, there is no chance of building a worldwide alliance that could deal with poverty and inequality and climate change and financial instability.”

A spokesperson for Corbyn’s campaign said: “It is necessary to be credible but credibility cannot mean an orthodoxy of austerity that chokes off recovery. We need a Labour Party that stands for growth, investment and innovation across the whole country.”

Unions have piled in behind Jeremy Corbyn’s leadership campaign, despite his claims that it relies on small donations.

Corbyn’s campaign states: “We have no big private donors. Jeremy wants Labour to become a democratic social movement again, dedicated to real change. Only Jeremy will deliver that change.” Entries on Mr Corbyn’s House of Commons’ register of members’ interests, which was updated last week, show his campaign took in nearly pounds 100,000 between June 23 and July 31 from just four unions.

The train drivers’ union RMT and Aslef gave £25,000 and £10,000 respectively while the Transport Salaried Staff Association and Unite gave £6,000 in free office space. Unite also lent the Corbyn campaign £50,000, which has to be repaid on September 12, the day of the result of the leadership election.

Corbyn’s campaign has so far raised £120,000 in small donations and said it was now looking to “hit our new bullseye” of £180,000.

— The Telegraph Group Limited, London 2015