09/06/2015 17:24 BST | Updated 09/06/2016 06:12 BST

Labour Party 'Under Mortal Threat'

Leading Labour leadership contenders have pledged not to share a platform with David Cameron in any campaign to keep Britain in the European Union as they laid out their plans to rebuild the party from the grassroots in the wake of the general election defeat.

In frank exchanges with trade union activists, the five MPs made it clear how much work the party had to do to win back support from across the country, and what mistakes were made in the run up to last month's poll.

The five, Jeremy Corbyn, Andy Burnham, Yvette Cooper, Liz Kendall and Mary Creagh faced questions over the Labour-union link, MPs' pay, the benefit cap and the party's election manifesto, when they spoke at the annual conference of the GMB union in Dublin.

Everyone except Ms Creagh said they would not share a platform with the Prime Minister in the run up to the promised referendum on the EU, an issue many believe cost the party votes in Scotland when Labour politicians joined Conservatives at meetings and rallies against Scottish independence last year.

Ms Kendall said the party was under "mortal threat", adding: "The scale of the defeat means we must change or face irrelevance."

Mr Corbyn, the most left-wing of the candidates, told delegates his purpose in standing was to raise issues about how to face austerity and the way the Government will treat people over the next five years.

Mr Burnham said Labour had lost touch with many supporters and was seen as a "Westminster elite", talking in "political code".

"I will take Labour out of the Westminster bubble and will lead a party that people can relate to," he said.

Ms Creagh said Labour was trusted to run the NHS, councils and schools, but parts of the electorate did not trust it to run the economy.

"Labour needs a fighter - and I am a fighter," she said.

Ms Cooper said Labour's election campaign was too "narrow", adding that the party should reach out to win back voters "left behind".

Mr Burnham and Ms Cooper were jeered by delegates for failing to give a yes or no answer when they were asked if they backed Conservative plans to reduce the cap on household benefits to £23,000 a year.

Both Ms Kendall and Ms Creagh said they were in favour of the change - to ensure it was impossible to receive more in welfare payments than the average family earned from work, though they warned it could cause localised problems.

To resounding cheers, Mr Corbyn was the only one to declare himself opposed, insisting that in the absence of regulated private rents it would result in "social cleansing" in central London.

But while Mr Burnham said he backed the principle of the cap, he insisted it was "unfair" to expect such a direct answer.

"I am not setting my face against changes to the benefit system but it depends how they do it and I am not going to give you an answer like that to a question that complicated," he told Mirror journalist Kevin Maguire, who was chairing the session.

Ms Cooper also faced dissent from the floor as she declined repeated requests to answer yes or no, despite saying the current policy was "unfair" and warning of damaging implications.

"I understand that everybody wants a yes or no, but we need to reform the legislation. As it stands, I think it is unfair.

"It is our job in the Labour Party to try and argue for change and to try and make changes, whether that is in Parliament or anywhere else and that is what we are going to have to do."

All five candidates said they did not believe Labour's election manifesto was too left wing, with Mr Burnham praising former leader Ed Miliband for the policies the party fought on.

Asked by Scottish delegate Duncan Walker how Labour could win back votes in Scotland, all five agreed the party needed to re-connect with voters.

Ms Cooper said Labour had to be a party for the whole country, adding: "We have to rebuild - but it will take us time."

Mr Burnham said: "I would say that it is the best manifesto that I have stood on in the four general elections I've stood for Parliament for Labour.

"I pay tribute and give credit to Ed Miliband. I believe he did something important in re-focusing our party on inequality."

Ms Creagh stressed the need for major investment in the transport infrastructure.

"We had a huge row about High Speed Two. I want High Speed Three, Four and Five. I want Crossrail Two, Three and Four.

Physical mobility is key to social mobility," she said.

North West and Irish delegate Kevin Flanagan asked the candidates - to loud applause - if they were in the toilet when Labour's election manifesto was drawn up.

Mr Burnham said he was not as involved as he would have liked.

He repeated that he had decided not to accept any donations from unions for his campaign because he believed it would put him in a stronger position to defend the Labour/union link.

In contrast, Ms Creagh said she would be happy to take donations from unions.

Ms Kendall said she would have liked to see the manifesto contain pledges to help elderly people.

Mr Corbyn said there was an issue about democracy in Labour since the role of the party's annual conference been reduced in favour of policy forums.

Ms Cooper said many policies in the manifesto were good, such as the planned changes to zero hours contracts, and pledges to scrap fees for taking cases to an employment tribunal.

The five candidates were asked a series of specific questions to test their knowledge of the price of goods and services.

Ms Creagh correctly answered how much a TV licence cost, saying she paid by direct debit; Mr Corbyn thought a prescription in England cost £7.60, rather than the correct figure of £8.20; Ms Cooper thought the minimum wage for apprentices was under £3 an hour (it is £2.73).

Mr Burnham incorrectly guessed a litre of petrol was £1.60 rather than £1.16 and Ms Creagh knew how much she paid for a loaf of bread (£1.25).

Mr Corbyn received the loudest applause in the hall, such as when he pledged to attend an anti-austerity rally in London on June 20, but delegates said later there was no stand out winner during the two-hour long hustings.

Many believed Mr Burnham and Ms Cooper strengthened their positions as front runners in the election.

Nominations opened today, with the five candidates needing the support of 35 MPs to stand.

The GMB executive will meet later in the summer to decide whether to recommend a candidate during a ballot of members.

The result of the election will be announced on September 12.